Found more info on this issue.
There are a lot of places in the code where a safe handle object is created and not disposed.
All the calls to GetPredefinedSymbol leave a safe handle object (SymbolicExpression) behind.
Also, InternalString is created without being disposed and InternalString.GetInternalValue is called without disposing.
Probably there are more.
All these leave behind undisposed objects, and then, if the app domain is unloaded, sometimes the engine finalizer is called before these objects' finalizers leading to this error.
So now simple operations like getting an entry from a CharacterVector leave behind several undisposed objects.
This brings the question why GetPredefinedSymbol is called again and again for the same values instead of caching the results.
There are a lot of places in the code where a safe handle object is created and not disposed.
All the calls to GetPredefinedSymbol leave a safe handle object (SymbolicExpression) behind.
Also, InternalString is created without being disposed and InternalString.GetInternalValue is called without disposing.
Probably there are more.
All these leave behind undisposed objects, and then, if the app domain is unloaded, sometimes the engine finalizer is called before these objects' finalizers leading to this error.
So now simple operations like getting an entry from a CharacterVector leave behind several undisposed objects.
This brings the question why GetPredefinedSymbol is called again and again for the same values instead of caching the results.