I was using the python fifo client to generate full and incremental backups as explained in https://blog.project-fifo.net/backups-with-project-fifo/.
Creating the full monthly backup worked fine. The first two incremental backups worked as well. A "Snapshot creation failed!"-error occurred while generating the third incremental backup.
The only error that I could find was in the howl logs and is attached to this ticket.