Oh hey, I understand constraint sets! Few people do, I imagine. I use them all the time for kinematics. Sometimes you want a joint that you can drag in realtime. You can even change the "regen value" with a family table or flexibility, to change what position it pops to upon regeneration. But, this isn't great for all use cases. Sometimes you want a joint locked to a given position, and want to be sure it'll come up in that position when the file is opened, without a regeneration. So, you need rigid constraints, not kinematic. By setting up two constraints for the same part, you can get both functions. You can pick which set is active via family table, or create a relation to handle it and control that. Lately, I've been creating a "HOME_ANGLE" parameter, and using relations to apply that to both the kinematic joint regen value, and the orientation of a CSYS used for the rigid joint. Another parameter controls which of these sets is active, and that can be set via family table or flexibility. (i.e. "if JOINT_LOCKED == yes"...)
The trick is that to find and control these values, you need to look at component level parameters. From the parameter dialog, change the "look in" menu to "Component", click the arrow, and then click the desired component from your model tree. It'll have a string parameter called "PTC_CONSTRAINT_SET", which lists the active sets separated by commas. (In rare cases, you may need more than one active at a time.) Inconvenient thing about changing these by family table or otherwise, is you have to type the names in exactly, there is no pop-up menu or anything that will list the options available.
I've never seen the particular error you mentioned, but I'd guess that someone changed the name of a set or deleted one, and something is trying to activate it. Check relations or family tables.
Relations can possibly be buried at the component level too. So identifying the particular component that is giving you trouble would be helpful. As others have said, you could include one model at a time, until the error appears...
EDIT: just realized this is a 2 year old question, so my answer probably isn't useful to original poster... It was at the top of the list because another comment had also been added today, seeking clarification of prior advice...
