Another day, another quick fix - I hope it helps!
The error “For security reasons DTD is prohibited in this XML document” does not help much…
Google suggested me to change my DNS entries to 220.127.116.11 or 18.104.22.168 - in my case that had no impact at all. Still, my assumption was (Occam’s razor, actually), that it has something to do with DNS - to many blog posts I read said something with DNS.
Easiest way to verify? Fiddler, right!
So in my case it was a combination of my ISP and IPv6 - I changed my DNS on my PC and disabled IPv6 - and everything is back to green!
That post pushed me in the right direction: Office 365 – DTD is prohibited in this document issue - thanks a bunch!