Firstly which release of Monarch V11 do you have?.
11.5 was quite a significant change.
That said most models should still work unless perhaps you have some really old .mod files.
It's a while back now but I seem to recall that at some point direct conversion of certain very old .mod files was no longer available (on the basis that things had changed over the years and very few people were likely still on the old models or had not already upgraded them during previous version updates along the way.
Putting that aside, once into V11 things like MS Windows .NET versions come into play (and associated patches and updates).
The error is a generic Microsoft error message - it could perhaps be more helpful!
If you search for the message (as you probably have) you will likely get a very large number of hits.
I'm fairly sure it has been discussed here previously but whether a forum search will find a useful reference quickly that is in line with your specific problem I cannot be sure.
Tell us which release of V11 you now have and we can take it from there.