It worked! Thank you so much - you’re my hero 
I already tried renaming my project folder before asking this question. But I tried starting my XCode project then (as we’re using C++). That didn’t work out. Opening the uproject did the trick. How could I not think of that facepalm.
I’m still not really getting why renaming the folder helps… but the external references are gone, so I got that going for me 