Ok, trying to narrow the problem down, i upgraded to the latest version. I still get some errors moving files. It doesn't seem to be related to zfs or the file system. I think it has something to do with the no memory error that occurs. Once this error occurs, then your unraid starts behaving badly.
in this case, this memory error popped up when i tried to move a few thousand files from one folder to another folder, through the unraid file manager.
Once this occurs, the move job stops working (but with no error in the gui). You no longer can do any file management, as it will say "a job is running"
And rebooting the system seems to be the only way to fix it.
There seems to be some sort of bug or some error handling that needs to take place so it doesn't lock up the file manager, when this occurs.