It's a select objects problem. Alt MouseLeft=select+ should get an enveloping square appear,
then many objects get to flash instead of just one. Copy works after that for all flashing.
( In Builders Unlocked [410 for me] they turn red instead of flashing ).Lucky for you I had SAS MissionProCombo running for another question ( by creature ) then found
I had the similar problem as you did.
I deactivated SAS MissionProCombo then select/copy many objects in FMB now works OK again.