We received "Bad Request - Request Too Long" error when we tested multiple user account on a single computer.
We just try to log-in by Account1 and log-out. Try to log-in by another account (e.g. Account2), repeated this sequence for 10s of accounts.
We saw this problem on Microsoft Edge and by removing cache on Edge, the problem has gone.
We believe this is a kind of bug and hopefully resolved in the future.