Please check the user rights of the folder upload.
Searched around the forum, found this thread.
Composer plugin was disabled for some reason.
Activated & rebuilt NodeBB.
The 24h delay was the issue.
sudo apt-get install build-essential will fix it.
Thanks you so much!
I still don't know what caused everything to turn bad.
I solved this by installing SSL as that allowed the ws connection to work properly through the ISP in question.
It seems that there is still the possibility to upload a profile cover picture, even though I disable "upload images" in privileges for registered users.