When trying to update the firmware of a PS6510, I get the following error message:

Array firmware update from version V9.0.7 to V9.1.1 failed. Reason: You cannot upgrade to this firmware version because of compatibility issues. Update aborted.

Can anyone explain why this is the case?

Current situation is one member with 9.0.7 firmware, three members with 8.1.10 firmware. 

Thank you in advance,

Erik