The error number -28 is from the shared memory dumps.
After we repeated the restore a few times the file system for the dumps
was full.
But the chunks are located somewhere else. Sorry for sending this
confusing part of the message log.

After this we made a new backup/restore and then it worked.
But this makes it even more confusing for me, because it seems to be a
sporadic problem.

If it is not officially supported, it makes probably no sense to get of
the bottom of this problem.