I posted around about it, trying to figure it, ripped apart the source code to the point where I found that manually decrypting the data was resulting in gibberish. All my backups were the encrypted data, so those didn't really help either.
I still don't know what happened to this day, but I've been eternally wary of personal fileservers ever since.
[0] https://owncloud.org/blog/how-owncloud-uses-encryption-to-pr...