ISO Storage - Storage domain not in pool

I have an NFS ISO storage. It's active on one of the two data centers I got. 

Trying to activate it gets me this error:

Storage domain not in pool

(Error code: 353)

I don't really get the error message. I have verifed that the hypervisors in that data center can reach the ISO domain host via NFS.

Where should I look? Rhevm.log on RHEV Manager doesn't really help me.

Looking in /var/log/messages on the hypervisor show this:

kernel: NFS: Cache request denied due to non-unique superblock keys