It sounds to me like it might be a constraints problem. It sounds like the design is overconstrained or you might have conflicting constraints. So I would start by reviewing the constaint files.
In reply to Scrivner:
In reply to potat0e:
Running in multimode means that instead of checking for discrepancies within a single set of constraints, you will have to check between all sets of constraints that are used in the multimode. I would say that running multimode makes it more likely that there is a conflict in the constraints between different modes.
I was going to suggest you run in MMMC, since that should prevent the problem you're describing. Make sure that all your setup views and hold views are active when you do your optDesign calls.
In reply to Kari: