since a few weeks I have the same problem that has been described for the MacOs version already in combination with OneDrive. Just that I have it with Windows and WebDav. It stopped syncing returning the error, that the password of data on WebDav is required. Before everything worked fine, now it doesn't. First I thought it might be my fault but after trying several different approaches I think it's rather not.
I deleted the whole sync setup and deleted the data on the WebDav share. Setting it up again and syncing works fine, until I change some data (like adding a new login) then it starts failing. The same goes with the Android version. It seems changing data is corrupting the files.
Android Version: 184.108.40.2063
Desktop (Windows) Version: 6.4.3 (666)
WebDav: nextcloud (current version 19)
I hope you can help me to resolve this issue.