I tried debugging, but there are no logs on my side. Raised an issue on the GTS project and one of the GTS maintainers replied:
Hiya, if GtS is redirecting you back to the app’s redirect_uri then there won’t be any helpful logs because as far as GoToSocial is concerned, everything works as expected. Maybe trakt.tv doesn’t like something about the code or state that’s being provided in the query parameters, but I’ve no clue what it might be objecting to, or why. Is that an open source project, do you know? If so we could just have a look at the code for handling callbacks and see what it’s trying to do.
Is there any chance to see some underlying cause what gives the “Invalid credentials” error? As I mentioned, every other app I tried (I think it’s at least 5 now of different types) authorizes without any issues 
If you’re up to it, I’m willing to get on a call to help debug. Or I could create a temporary account on my instance and send you credentials 