POSSIBLE REASON:
Something in the user's dashboard might got corrupted.
HOW TO VERIFY:
To identify if the dashboard as being an issue, try to log into something that isn't the dashboard, e.g. following a direct link to an experiment.
Any other user in the system can go to e.g. an experiment (one the original user has access to) and can copy the URL out of the browser. Also, if any experiment was shared with the affected user, or that user was mentioned in a comment, then they were sent an email with a direct link too.