Backup generates and renames backup file but deletes it (Nextcloud)

• I have searched open and closed issues for duplicates
• I am submitting a bug report for existing functionality that does not work as intended
• I have read https://github.com/signalapp/Signal-Android/wiki/Submitting-useful-bug-reports
• This isn't a feature request or a discussion topic

Bug description

For external storage I chose a folder in Nextcloud. This results in the storage location being displayed like "-123456789/12345" and the backup failing with "Renaming temporary backup file failed!" according to the debug log. I have no SD cards.

See also:
https://github.com/signalapp/Signal-Android/issues/7763
https://github.com/signalapp/Signal-Android/issues/7692

It appears the file is successfully created and renamed, but Signal decides to delete it anyway, before Nextcloud can upload it:

Nextcloud app:

Nextcloud web interface:

Steps to reproduce

Turn on external backup. Select Nextcloud folder. Run backup.

Actual result: Backup takes a while and eventually it stops and says "Last Backup: Never".
Expected result: The backup completes successfully or gives me an informative error message.

Device info

Device: Xiaomi Mi 9T (davinci)
Android version: 10
Signal version: 5.0.0
No SD cards mounted.

All permissions for write access are granted.

No source file, test, or entry point is named. Begin with the external-backup reproduction steps using a Nextcloud folder and inspect the backup behavior around file creation, renaming, and deletion. Done means the backup completes successfully or reports an informative error instead of ending with “Last Backup: Never”.