Horde login problem

Has anybody seen something like this before? I have Horde and Horde IMP installed and have had users using it occasionally since last Fall. Recently it has started not going past the login screen. According to the log file the login is successful, but the WebUI just goes right back to the login page.

After a bit of googling I found some comments about cookies and sessions, so I deleted the cookies for the site I had in my Firefox browser and the login now works correctly for me using this browser. However I tried to also delete cookies for IE on my other machine and it still has the login problem.

Any ideas?