I'm not sure exactly what can be done to fix it, but if you have existing cookies from the site (e.g. you have previously authenticated with Facebook) it is very likely that authenticating with Google will fail with "401 Unauthorized". Unfortunately there is very little information on the error page and most users would give up at that point assuming the site is broken.
At the very minimum the error page needs to explain that clearing your cookies will fix it. Better would be to proactively detect the situation and transparently fix up the cookies so that authentication can succeed.
I'm not sure exactly what can be done to fix it, but if you have existing cookies from the site (e.g. you have previously authenticated with Facebook) it is very likely that authenticating with Google will fail with "401 Unauthorized". Unfortunately there is very little information on the error page and most users would give up at that point assuming the site is broken.
At the very minimum the error page needs to explain that clearing your cookies will fix it. Better would be to proactively detect the situation and transparently fix up the cookies so that authentication can succeed.