[Bug]: 3.14.0 does not create private.key for E2EE when initializing E2EE

### ⚠️ 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

After resetting E2EE for a user (starting from scratch) when trying to enable E2EE on the 3.14.0 endpoint, the button "set up encryption" (translated) does nothing. In the back, a `Username.public.key` is created, while a `Username.private.key` isn't. That leads to a strange half-setup E2EE with the UI at some point (after restarting the client) allowing to show the seed, where there's not even a box (no seed shown).

Server log spammed with

```
[end_to_end_encryption] Warnung: Could not find the private key of the user: Username
GET /ocs/v2.php/apps/end_to_end_encryption/api/v1/private-key
von xxx.xxx.xxx.xxx von Username um 25.09.2024, 15:00:19
```

I spent two hours on trying to fix this along with another 3.14.0 specific issue (https://github.com/nextcloud/desktop/issues/7204). E2EE with NC is annoying as hell as it breaks every few weeks or at best months. I'm now at a point where I simply can't trust any NC (endpoint) update anymore. No idea how 3.14.0 could be released with so fundamental issues not fixed (not tested enough?).

No idea how 3.14.0 could be released with so fundamental issues not fixed (not tested enough?).

Solved by downgrading to 3.14.0, like it works for https://github.com/nextcloud/desktop/issues/7171 too.

### Steps to reproduce

1. Reset E2EE on server
2. Remove E2EE from Windows endpoint running 3.14.0
3. Try to set up E2EE on the 3.14.0 Windows endpoint - pressing the "enable E2EE" will do nothing

### Expected behavior

E2EE can be set up as expected. At least SOME response should be given.

### Which files are affected by this bug

none

### Operating system

Windows

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

Windows 11

### Package

Official Windows MSI

### Nextcloud Server version

28.0.10

### Nextcloud Desktop Client version

3.14.0

### 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
[end_to_end_encryption] Warnung: Could not find the private key of the user: Username
GET /ocs/v2.php/apps/end_to_end_encryption/api/v1/private-key
von xxx.xxx.xxx.xxx von Username um 25.09.2024, 15:00:19
```

### Additional info

Should be mentioned at https://github.com/nextcloud/desktop/issues/7191.
Tested with two Windows 11 endpoints, one with normal sync and one with VFS sync, both with same behaviour/output.