I typically have a couple of dozen runtimes attached at any given time. Poser 12 does not have any problem with this...except when the scene was created with things from runtimes that aren't attached. Attaching the missing runtime(s) fixes the problem.

Penelope, where do you have your DAZ figures installed? If they are in the Poser 11 runtime, you have to attach that runtime to Poser 12.