That message isn't the actual error but the standard footer added to all Fisheye and Crucible pages and error screens. When a login session crashes or has issues, this footer is often the only visible text.
I would:
A corrupted browser cache or session cookies frequently causes the login page layout to crash. If an incognito window works, clear your browser's history and cookies.
Look directly above that text or press F12 to open your browser's developer console. Look for the actual underlying code and errors (such as an HTTP 500 Internal Server Error or a timeout).
If it is a server error, you will need to inspect the atlassian-fisheye.log file on the server at the exact timestamp of your login attempt to see what blocked the request.
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