This happened for me and for the two other members of my team. Upon logging in for the first time to our new Jira Cloud setup, we cannot escape from the "Welcome" sequence that attempts to show you the differences between Projects, Issues and Tasks. It appears to happen on this URL:

https://ext-tickets.atlassian.net/secure/WelcomeToJIRA.jspa

This bug happened in both Chrome for the Mac and Safari, both most current versions. I was able to stop it by trying to time a button click during the refreshing to get to another URL. Other users may not know to try that and never get past the Welcome process.