Update - A fix to 400 error should be deployed in the next few days. the fix will be part of MDI 2.168 version.
Jan 9, 11:24 UTC

Identified - Hi All,
please note we have identified an access issue to our main portal.atp.azure.com that might result to some of you with error 400 due to cookie size, the issue is under investigation and as temporary workaround we ask to please use your dedicated workspace portal URL
Dec 2, 11:57 UTC