can't switch the existing feed url to a different feed url

I encounter the same issue! I have the message “This XML document is invalid, likely due to invalid characters. XML error: junk after document element at line 128, column 250” on the widget, still the page based on the feed still works, as can be seen here:
https://www.fian-berlin.de/themen/presse/

Thinking the problem was on the feed’s source site I tried another feed url, but when clicking on “Use this feed” it gives me this blank page “Are your sure you want to do this” (in my case in German) without any possiblity to confirm.

In my case it says that the it’s returning since 15 hours ago, so it seems not to be a problem with the update as it worked fine last week.

… i have removed the widget for now, as it’s irritating for my visitors to have the xml error… but the issue should be understandable w/o having it, or, dear dev, let me know if you want to troubleshoot and i’ll put it on again.