Opening an office file with WebDAV mount via NC corrupts the file

I’m mounting NC via WebDAV on Windows 10. It occurred suddenly on June 15.
We only use either Word or Excel.

When the user tries to open the file, the following dialog will appear.

I found a problem with some of the contents of “(FILENAME)”. Do you want to recover the content if possible? If you trust the publisher of the book, click yes.

Some files cannot be opened with YES.

The workaround I’m aware of is to download it locally once via FTP. Then upload it to NC. But this seems to cause the same problem again after some time.
I’ve uninstalled it, suspecting a recent regular Windows 10 update, which It doesn’t seem to matter. I face the same problem on a PC that has not yet been updated.
It’s not an exaggeration to say that the problem is no longer some specific files, but all of them.

Currently, the administrator downloads it via FTP and passes it to the user. Then the user edits it and returns it to the NC.

The problem with this office file is that it has problems. Is there a solution to the problem?

Nextcloud 19.0.0, PHP 7.3.18, nginx 1.16.1

I’ve been getting the following errors on a minute-by-minute basis since I updated to NC19. Does this affect me?

[PHP] Error: session_start(): A session had already been started - ignoring at /MY-NEXTCLOUD/lib/private/Session/Internal.php#209

PROPFIND /remote.php/dav/principals/users/USER/

Is there any way to stop this error?