First of all: welcome to the forums and nice choice to go Modeler. I’ve had a Modeler license for years now and I think it’s seriously worth it.
I’m not sure but I do have a good idea what could have happened.
First of all: the amount of supported features differs per license. The trial / evaluation lets you test things using the Enterprise features whereas Modeler has significant less features in direct comparison to Enterprise. So it is definitely possible that some things may no longer be available right now. However… If you open a VP project using a “lower tiered” edition than the original (so, for example, from Professional to Modeler) then the lower tier will be able to access the whole project, but some of the diagrams which it doesn’t support can only be accessed in readonly mode (so “viewing only”).
As to your problem: you mentioned referenced projects. Did you actually add project references? So: “linked” 2 projects together?
This is actually a feature of VP Standard edition and up, but it is not supported by the Modeler edition. So if you did indeed reference (or “link”) other projects during your trial then this would explain why that no longer works: because this option isn’t available anymore.
Your best option as far as I see it is to open the projects individually.
I hope this can help!