Daniël I had the same problem a few weeks ago, at some point all actions simply stopped working, and solved it by reauthorizing OAuth. At the time I thought it was caused by a change bunq implemented for OAuth authorizations, but since I just tested it works fine for me today, this cannot be the reason it doesn't work for you.
Was it a one-time error, or does everything fail?
PS: if you use the reply function, I will see your question faster...