"Cannot decrypt this file, probably this is a shared file. Please ask the file owner to reshare the file with you."
when I try to download, to sync or to view some files I have added in the past months.
I have never enabled encryption and occ verifies this. The files are also not shared, they are my own files.
My Nextcloud instance is really old, maybe back to the ages of owncloud. Is there anything I need to do to fully disable encryption? I remember there was similar error messages popping up from time to time but never faced data loss like now.
You seem to have overlooked the support template. Can you post the entire stack trace / raw log entry?
That error message is generated by the Encryption app itself, so at a minimum that app is enabled in your environment for some reason (it’s not enabled by default).
when I try to download, to sync or to view some files I have added in the past months.
Does it happen if you create a brand new test file today? I ask because it’s possible that encryption was enabled previously - but is not now. Unless decrypt-all was used, there would be encrypted files left around from that time period.