Try a different browser.
I had the same issue. Try going to a different desktop, log in and pair there. Worked for me.
I had the same problem. After the 4 or 5th time reentering the Strava info the connection seemed to work.
With my it’s okay now, i have done the syncing via my pc in place of my Smartphone
and now it works !
Strava has had server issues all week. It is posted on their site. Mostly impacting mobile app and syncing with other apps.