Strange Unresolved external symbol errors. Seems to be triggered by increased code length?

After 2 years working on my c++ project, I suddenly have a weird compile error.

It seems as if the length of the code in my cpp affects whether the project compiles or not.

Adding any significant amount of code lines or even commented text lines causes the Unresolved external symbol errors to appear.

here are the errors

In your class UMyBPFunctions do you include the header to ANpc_Actor_RSClass?
Is ANpc_Actor_RSClass a static class used in UMyBPFunctions?

Hi :), yes it is included
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Im not sure what you mean by static class. It is a actor class derived from ue4’s actor class.

Within UMyBPFunctions I am interacting with it and it’s functions and variables.

It seems the error stems from UMyBPFunctions calling ANpc_Actor_RSClass. Could you show you includes in that library and the part where you have any interactions with the class? (spawning or modifying as a ref etc)

Is it’s header inside of UMyBPFunctions cpp file?