[Bug]: Login failed - Desktop client - MacOS VFS

### ⚠️ Before submitting, please verify the following: ⚠️

- [x] This is a **bug**, not a question or a configuration issue.
- [x] This issue is **not** already reported on Github (I've searched it).
- [x] Nextcloud Server and Desktop Client are **up to date**. See [Server Maintenance and Release Schedule](https://github.com/nextcloud/server/wiki/Maintenance-and-Release-Schedule) and [Desktop Releases](https://nextcloud.com/install/#install-clients) for supported versions.
- [x] I agree to follow Nextcloud's [Code of Conduct](https://nextcloud.com/contribute/code-of-conduct/)

### Bug description

The MacOS user adds files to sync, but files do not sync. The client reports "All synced!" and an error message appears in the nextcloud server logs. I've searched for other similar issues, but they all used LDAP authentication and are marked closed. This server uses the standard/default un/pw authentication.

The user can see other recent server activity within the client (new files added from other users).


IP and user name removed for privacy.
```
{"reqId":"Zqd97V2jN52mLstQYi8r","level":2,"time":"2025-04-14T21:53:11+00:00","remoteAddr":"000.000.000.000","user":false,"app":"core","method":"PROPFIND","url":"/remote.php/dav/files/user_account/","message":"Login failed: 'user_account' (Remote IP: '000.000.000.000')","userAgent":"Nextcloud-macOS/FileProviderExt","version":"31.0.2.1","data":{"app":"core"}}
```

This issue occurred after upgrading to 31.0.2. The only other MacOS user is not using VFS, but standard sync is working without issue for the second user.
...

### Steps to reproduce

1. Set up MacOS VFS client
2. Add files to the virtual drive
3. Observe server error
...

### Expected behavior

Files synced without login error
...

### Which files are affected by this bug

accountmanager.cpp, accessmanager.cpp, keychainchunk.cpp, webflowcredentials.cpp

### Operating system

macOS

### Which version of the operating system you are running.

Sonoma 14.6

### Package

Official macOS 12+ universal pkg

### Nextcloud Server version

Nextcloud Hub 10 (31.0.2)

### Nextcloud Desktop Client version

3.16.2

### Is this bug present after an update or on a fresh install?

Updated to a major version (ex. 3.3.6 to 3.4.0)

### Are you using the Nextcloud Server Encryption module?

Encryption is Disabled

### Are you using an external user-backend?

- [x] Default internal user-backend
- [ ] LDAP/ Active Directory
- [ ] SSO - SAML
- [ ] Other

### Nextcloud Server logs

```shell
{"reqId":"Zqd97V2jN52mLstQYi8r","level":2,"time":"2025-04-14T21:53:11+00:00","remoteAddr":"000.000.000.000","user":false,"app":"core","method":"PROPFIND","url":"/remote.php/dav/files/user_account/","message":"Login failed: 'user_account' (Remote IP: '000.000.000.000')","userAgent":"Nextcloud-macOS/FileProviderExt","version":"31.0.2.1","data":{"app":"core"}}
```

### Additional info

OS: Unraid 7.0.1
Nextcloud Docker
PHP: 8.3.19
MariaDB (mysql): 11.4.5

Seemingly relevent client log snippet
```
2025-04-14 14:53:13:298 [ info nextcloud.gui.macos.fileprovider.socketcontroller /Volumes/builds/29334/macos-29334/src/gui/macOS/fileprovidersocketcontroller.cpp:213 ]: About to send account details to file provider extension "user_account@nextcloud.domain.tld"
2025-04-14 14:53:13:298 [ warning nextcloud.gui.macos.fileprovider.socketcontroller /Volumes/builds/29334/macos-29334/src/gui/macOS/fileprovidersocketcontroller.cpp:223 ]: Not sending account details yet as account is not connected "user_account@nextcloud.domain.tld"
2025-04-14 14:53:13:318 [ warning nextcloud.gui.account.manager /Volumes/builds/29334/macos-29334/src/gui/accountmanager.cpp:526 ]: Failed to read proxy password to keychain "Could not retrieve private key from keystore: The specified item could not be found in the keychain"
2025-04-14 14:53:13:320 [ warning nextcloud.sync.credentials.keychainchunk /Volumes/builds/29334/macos-29334/src/libsync/creds/keychainchunk.cpp:360 ]: Unable to read "user_account_clientCertificatePEM:https://nextcloud.domain.tld/:0" chunk "0" "Could not retrieve private key from keystore: The specified item could not be found in the keychain"
2025-04-14 14:53:13:321 [ warning nextcloud.sync.credentials.keychainchunk /Volumes/builds/29334/macos-29334/src/libsync/creds/keychainchunk.cpp:360 ]: Unable to read "user_account_clientKeyPEM:https://nextcloud.domain.tld/:0" chunk "0" "Could not retrieve private key from keystore: The specified item could not be found in the keychain"
2025-04-14 14:53:13:321 [ warning nextcloud.sync.credentials.webflow /Volumes/builds/29334/macos-29334/src/gui/creds/webflowcredentials.cpp:464 ]: Unable to read client key "Could not retrieve private key from keystore: The specified item could not be found in the keychain"
2025-04-14 14:53:13:322 [ warning nextcloud.sync.credentials.keychainchunk /Volumes/builds/29334/macos-29334/src/libsync/creds/keychainchunk.cpp:360 ]: Unable to read "user_account_clientCaCertificatePEM0:https://nextcloud.domain.tld/:0" chunk "0" "Could not retrieve private key from keystore: The specified item could not be found in the keychain"
2025-04-14 14:53:13:322 [ warning nextcloud.sync.credentials.webflow /Volumes/builds/29334/macos-29334/src/gui/creds/webflowcredentials.cpp:505 ]: Unable to read client CA cert slot "0" "Could not retrieve private key from keystore: The specified item could not be found in the keychain"
2025-04-14 14:53:13:333 [ info nextcloud.gui.account.state /Volumes/builds/29334/macos-29334/src/gui/accountstate.cpp:498 ]: Fetched credentials for "https://nextcloud.domain.tld" attempting to connect
2025-04-14 14:53:13:333 [ info nextcloud.gui.account.state /Volumes/builds/29334/macos-29334/src/gui/accountstate.cpp:290 ]: check connectivity
2025-04-14 14:53:13:333 [ info nextcloud.sync.credentials.webflow /Volumes/builds/29334/macos-29334/src/gui/creds/webflowcredentials.cpp:117 ]: Get QNAM
2025-04-14 14:53:13:333 [ info nextcloud.sync.connectionvalidator /Volumes/builds/29334/macos-29334/src/gui/connectionvalidator.cpp:85 ]: No system proxy set by OS
2025-04-14 14:53:13:334 [ info nextcloud.sync.accessmanager /Volumes/builds/29334/macos-29334/src/libsync/accessmanager.cpp:75 ]: 2 "" "https://nextcloud.domain.tld/index.php/204" has X-Request-ID "6321c0c1-122b-4309-b702-faf5b5d905be"
2025-04-14 14:53:13:334 [ info nextcloud.sync.networkjob /Volumes/builds/29334/macos-29334/src/libsync/abstractnetworkjob.cpp:364 ]: OCC::CheckRedirectCostFreeUrlJob created for "https://nextcloud.domain.tld" + "status.php" "OCC::ConnectionValidator"
2025-04-14 14:53:13:962 [ info nextcloud.gui.updater /Volumes/builds/29334/macos-29334/src/gui/updater/sparkleupdater_mac.mm:303 ]: launching background check
2025-04-14 14:53:13:964 [ info nextcloud.gui.updater /Volumes/builds/29334/macos-29334/src/gui/updater/sparkleupdater_mac.mm:78 ]: Updater may check for updates: YES
2025-04-14 14:53:14:068 [ info nextcloud.sync.credentials.webflow /Volumes/builds/29334/macos-29334/src/gui/creds/webflowcredentials.cpp:406 ]: request finished
2025-04-14 14:53:14:068 [ info nextcloud.sync.accessmanager /Volumes/builds/29334/macos-29334/src/libsync/accessmanager.cpp:75 ]: 2 "" "https://nextcloud.domain.tld/status.php" has X-Request-ID "851a3595-c2d0-44dd-b141-81bba7e960e7"
2025-04-14 14:53:14:068 [ info nextcloud.sync.networkjob /Volumes/builds/29334/macos-29334/src/libsync/abstractnetworkjob.cpp:364 ]: OCC::CheckServerJob created for "https://nextcloud.domain.tld" + "status.php" "OCC::ConnectionValidator"
2025-04-14 14:53:14:110 [ info nextcloud.sync.credentials.webflow /Volumes/builds/29334/macos-29334/src/gui/creds/webflowcredentials.cpp:406 ]: request finished
2025-04-14 14:53:14:110 [ warning nextcloud.sync.networkjob.checkserver /Volumes/builds/29334/macos-29334/src/libsync/networkjobs.cpp:528 ]: No SSL session identifier / session ticket is used, this might impact sync performance negatively.
2025-04-14 14:53:14:110 [ info nextcloud.sync.networkjob.checkserver /Volumes/builds/29334/macos-29334/src/libsync/networkjobs.cpp:556 ]: status.php returns: QJsonDocument({"edition":"","extendedSupport":false,"installed":true,"maintenance":false,"needsDbUpgrade":false,"productname":"Nextcloud","version":"31.0.2.1","versionstring":"31.0.2"}) QNetworkReply::NoError Reply: QNetworkReplyHttpImpl(0x600002c79ce0)
2025-04-14 14:53:14:110 [ info nextcloud.sync.connectionvalidator /Volumes/builds/29334/macos-29334/src/gui/connectionvalidator.cpp:132 ]: ** Application: Nextcloud found: QUrl("https://nextcloud.domain.tld") with version "31.0.2" ( "31.0.2.1" )
2025-04-14 14:53:14:110 [ info nextcloud.sync.connectionvalidator /Volumes/builds/29334/macos-29334/src/gui/connectionvalidator.cpp:292 ]: QUrl("https://nextcloud.domain.tld") has server version "31.0.2.1"
2025-04-14 14:53:14:110 [ info nextcloud.sync.accessmanager /Volumes/builds/29334/macos-29334/src/libsync/accessmanager.cpp:75 ]: 6 "PROPFIND" "https://nextcloud.domain.tld/remote.php/dav/files/user_account/" has X-Request-ID "a33888d5-db21-44b8-8805-9161f449b651"
2025-04-14 14:53:14:110 [ info nextcloud.sync.networkjob /Volumes/builds/29334/macos-29334/src/libsync/abstractnetworkjob.cpp:364 ]: OCC::PropfindJob created for "https://nextcloud.domain.tld" + "/" "OCC::ConnectionValidator"
2025-04-14 14:53:14:129 [ info nextcloud.sync.account /Volumes/builds/29334/macos-29334/src/libsync/account.cpp:835 ]: appPassword stored in keychain
2025-04-14 14:53:14:147 [ info nextcloud.sync.account /Volumes/builds/29334/macos-29334/src/libsync/account.cpp:835 ]: appPassword stored in keychain
2025-04-14 14:53:14:171 [ info nextcloud.sync.credentials.webflow /Volumes/builds/29334/macos-29334/src/gui/creds/webflowcredentials.cpp:406 ]: request finished
2025-04-14 14:53:14:171 [ info nextcloud.sync.networkjob.propfind /Volumes/builds/29334/macos-29334/src/libsync/networkjobs.cpp:658 ]: PROPFIND of QUrl("https://nextcloud.domain.tld/remote.php/dav/files/user_account/") FINISHED WITH STATUS "OK"
```

Full Desktop Client log:
[sanitized_20250414_1453_nextcloud.log](https://github.com/user-attachments/files/19744145/sanitized_20250414_1453_nextcloud.log)

client settings