Failed to add Storage Domain xyz.
One of the rather easy to fix reasons might be a permission problem: an initial Gluster exported file system belongs to the user root. However, the virtualization manager (ovirt-m bzw. RHV-M) does not have root rights and such needs another ownership.
In such cases, the fix is to mount the exported volume & set the user rights to the rhv-m user.
$ sudo mount -t glusterfs 192.168.122.241:my-vol /mnt # cd /mnt/ # chown -R 36.36 .
Afterwarsd, the volume can be mounted properly. Some more general details can be found at RH KB 78503.