UrBackup Server crashed backupfolder was out of disk space

today I found my urbackup server not runnning any more. The backup folders had runned out of disk space. In /var/log/urbackup.log I got the following messages:

2018-11-28 22:22:57: ERROR: Could not free space for image. NOT ENOUGH FREE SPACE.
2018-11-28 22:22:57: WARNING: Not enough free space.

and then repeating for 4 minutes:

2018-11-28 22:26:25: ERROR: Error while writing compressed data to file
2018-11-28 22:26:25: ERROR: Writing to file failed
2018-11-28 22:26:25: ERROR: Last error: 28

After that nothing more.

Server Version is 2.2.11
OS: Debian 9.6
Filesystem of backupfolder: Btrfs