Scouts have mentioned getting this screen fairly often when they try to log in. I've seen the issue as well. A simple refresh usually fixes it but would be better to eliminate the issue if possible.
I've seen this too, once or twice. It seems that the user is logged in properly, but maybe the error happens during the redirect. Like you say, refreshing the page solves the issue.
Scouts have mentioned getting this screen fairly often when they try to log in. I've seen the issue as well. A simple refresh usually fixes it but would be better to eliminate the issue if possible.