This is not compiler error, this is UnrealHeaderTool crashing. UHT is a tool that parse UCLASS(), UPROPERTY() etc and generate extra code that register class and it’s elements to reflection system so engine see them exist (since when you compile C++ code all names and such are turn in to anonymous memory addresses, this is why this system exists for). When you remove UCLASS() you simply making UHT not to parse it at all, but UCLASS() and UHT generated code is required by UObject classes (this includes actors) as engine need to see them in order to manage them properly.
Since UHT don’t return specific error here you might hitting a bug here, can you paste how you declere that class that cause the, UCLASS() and first portion of it (but best would be whole class decleration)?