Some files will not synchronize


i have a big problem on our nextcloud 15 installation; We are using nextcloud since 3 months an all works fine… expect one thing: A user come to me this week and said he uploaded a file using WebDAV and Windows-Explorer and the next day the files was not there anymore! I checked the logging at the admin-panel but there is nothing unusual…
I have no idea why this happen… Maybe he had no/broken/failed internet connection during the copyprozess and after the connection was stable again the files were not synchronized? Is that possible?

Have anyone the same problem or is this a known issue?

thank you for helping
Kind regards Dominik