Sorry for bothering you with something that you are most probably very aware of:
the engine probably still misses a certain file. Maybe a model, maybe an external skin texture of a model.
One test, that you can do: put the exe and the relating files (mainly the ) into the project folder and start it from there - you should make a back up from that folder first, though.

About the point, that any version above 7.50 refuse to work with the game:
maybe, you use a certain entity in an unusual way, and the format of the entity has been changed since 7.50. I experienced something similar when switching from A6.341 to A6.60, because I used an animated terrain with a shifting texture as a river, saving the hmp as an mdl did the job.

(Even somehow senseless suggestions can help to get an idea how to solve a problem. That's the only reason that I dare to answer! )