Amazing, it worked. Lucky I still had this thread still open in
Firefox, I couldn't find this message by searching, it is not recorded
in my list of posts for this month nor does it show up in the message
list.
A few hrs ago it was the same (endless redirect) for Firefox. The
problem persisted for 2 hrs and then disappeared by itself, no cookies
cleared. All this time Chrome accessed Groups w/o any problem. Can be
a selective sabotage from Google by days and by UAs to incline extra
users to try Chrome instead of their current UA.