I've seen this before and short of removing all members and then resetting the last remaining functional unit, the only way to do this requires assistance from support. Have them do a remote session to your desktop and show them the group manager. Hopefully that helps them understand the problem.
We have two PS400e units in a pool, one of which failed several months ago.  After working for several hours with tech support we finally gave up and just recovered the data we had from tape, then reconstructed the VMs we lost.  The "good" unit was removed from the group manager and reconfigured, then added back in today, but the unit that had failed cannot be removed from the group manager. At this point I need to remove the system from the group so that I can reconfigure the hardware and add it back in.  I tried this in the GUI today and received an error message about deleting an offline member and "contact tech support." I placed the requisite call to tech support, but I don't think the technician understood what I'm trying to do.  The unit is not usable at this point because we can't reconfigure it and the pool sees it as "offline".  All I want to do is remove its instance from the group manager so I can factory reset it and add it back into the group, then reconfigure our pools so that we aren't spanning data across the two units any longer. Does anyone know a way to remove the "offline" unit from the group manager so I can reuse the system name and IP addresses?
Removing an offline member from a group/pool