Thanks!
Turns out it is actually WPML causing the problems, but I don’t know why. Wer are using the wpml_permalink hook to properly request the translation links, yet it does not work as it should, it returns gibberish with no reason. This is actually not the first time we are having issues with it, in fact this is now at least the 10th solution we had to implement.
Anyways, I have made yet another “bypass” to maybe resolve this issue as well. I really hope this is going to work now as it should by default. Let me know if it seems okay and I will push this to the upcoming release 🙂