Hi MAX2479,
I am attempting a similar setup using RDMs for MSCS across physical boxes. I add the RDMs to the first VM on Host1, then when I go to add the RDM to the second VM on another host by choosing "Use an existing virtual disk" the same error comes up. My shared storage is iSCSI SAN so I am not sure if this is supported with iSCSI SAN vs FC SAN. Let me know what ended up working for you.
Cheers