The mount points you are trying to mount remotely are not Unraid shares and you are setting the export rules manually for those shares. This is an Unraid share entry in the exports file:
"/mnt/user/isos" -fsid=105,async,no_subtree_check *(rw,sec=sys,insecure,anongid=100,anonuid=99,all_squash)
I suggest you take a look at this entry and compare to your entries and see if changes are needed. I suspect a permissions issue that UD is having trouble with. Check if 'all_squash' makes a difference.
As a test, share a normal Unraid share with NFS and try to mount that and see how it goes.
I fully understand that you can mount those with other systems. The issue is that the UD NFS client is probably more restrictive about permissions.