A client did its first full file backup, 133GB were transferred out of 260GB, which took a few good hours.
Then I upgraded the server software version.
After restarting, the client started the backup from scratch, completely forgetting the previously transferred 133GB of good data.
The server log shows lines like this:
2018-03-02 22:18:11: WARNING: File entry with id 1499089 with filesize=3889152 hash=A5X61oI5znw6i+MjIfD+Ig== not found for clientid 2 in file entry index while deleting, but should be there. The file entry index may be damaged.
And the web interface shows:
Why were hours of backup just thrown to the trash?