This is a difficulty I have never seen before. I set up a course with a lecturer and TA's. I gave the ta's logins and initial passwords. One of the ta's came to me to tell me that she could not log in. I used her login and password to log in on my computer. However, when I went to the laptop that she was using and did the same thing a message that said something like "you cannot use another person's login" appeared. How do I handle this?
I've seen this happen before, and have not been able to track down the exact circumstances. It seems to happen after logging out as a different user, for example, after logging out of a practice account. Using the "bread crumbs" at the top of the page to go to the login page again should take care of the problem, as those links don't include the user information that is causing the problem.
I've seen it before also. I suspect some kind of conflict with an existing cookie on the computer. Things to try -- (1) type in the url by hand (that gets rid of any "effectiveUser" clauses left over from having logged in previously). If that doesn't work (2) close the browser and start over -- I think that has always fixed the problem for me. If not (3) clear cookies from the browser and login.