Hi everybody, I have the same problem. I was on ownCloud 9.1 and recently upgraded to ownCloud 10.0.0.1 and a few days later I migrated to Nextcloud 12. As long as I sticked to the present ownCloud Client, everything was fine. On one Windows 10 System I installed the recent (2.3.1 build 8) nextcloud client without beginning a new sync of all files, just pointing the client to the old owncloud directory, no problems so far. Today on my Windows 8.1 System I replaced the ownCloud client with the recent nextcloud client. But this time, I started a clean new sync and a lot of files, mostly mp3-files provide the message, Christian_von_Glahn has already referred to. I also have a german system and the original message is: “Die heruntergeladene Datei entspricht nicht der Prüfsumme, das Herunterladen wird wiederaufgenommen”. Just in case someone looks for the german error message. On the Windows 10 System, I excluded an already synchronized directory from sync, so that the directory was removed by the sync client locally, then re-added the directory and now I get the same error message. So far I rebooted the whole Nextcloud server as well as the clients.
EDIT: I installed the last 4 owncloud-Clients down to 2.2.4 build 6408, the problem is still there. BTW after installing an older client, you get a message right away to restart the client in order to update to Version 2.3.2. I hope, that before the restart I really work with the old version. Question: Why isn’t there an option to deactivate automatic updates? What if I have an even more serious problem without data access at all, I think there should be an option to stay on the older version at least for a while.