Webdav access throws parse exception error

Hello. I am running seafile server on a Raspberry Pi. So far all worked great. However, I recently tried to sync data via the app “FolderSync” between seafile WEBDAV and my phone and now I get

org.xmlpull.v1.XmlPullParserException: Unexpected <! (position:START_DOCUMENT null@1:1 in dk.g0…)
as an error message in Folder Sync when testing the connection.

It has worked before on this phone. I may have had an uncontrolled power-off on the seafile server.
Any suggestions on how to get it running again?

Found a (rather strange) solution: I deleted seafdav.log and seafile.log. Afterwards the error message disappeared.

Unfortunately, the message appears again and this time the removal of the files does not do the trick