@ Joop Beris
SSH would not work and I really would not want that level of access anyway.
I changed my dev site to Dutch but I am unable to replicate this.
I suspect that when you switch your site back to Dutch the admin will be fine. So please do that.
After you do, do some common management tasks such add an entry and edited entry. The same for a category. Change some settings in Connections and save the changes. Do any action in Connections that you recall doing recently.
I suspect that after one of those actions the issue will reappear. Let me know which action. That should help me identify the exact string causing this issue for you.
I would have done this but I did not want to lock you out of the admin unexpectedly. let me know what you learn.