There may be a line break missing in your /etc/backup-data.d/custom.exclude.
The output of cat /etc/backup-data.d/custom.exclude should look like
[root@testserver ~]# cat /etc/backup-data.d/custom.exclude
/var/lib/nethserver/ibay/test
[root@testserver ~]#
So just open the file in mcedit and go to end of line and hit enter and save it…