Try running a windows disk clean up. I found this post from awhile ago.
HI Ray! Can you please check if there is anything coming from the Modeler in Event Viewer, under Windows Logs -> Application menu. Maybe there is a crash report over there.
For as far as I know there are no limits, I often have multiple branches and thus multiple modeler instances open on one laptop.
Did you try installing an older/newer modeler version and see if that one worked?
Also, maybe, a very obvious action but did you try restarting? Sometimes it is just Windows acting weird.
And then as a last question do you open the modeler directly, via the version selector or by clicking your projects MPK file?