[horde] Strange login problem

[Kevin Konowalec]

Horde: 3.1.4
Imp: H3 (4.1.4)

Since transitioning to the new versions of horde and imp we have been  
getting more and more complaints about login problems.  When you  
enter in correct username and passwords and submit, horde will  
frequently spit you right back to the login page without any errors.   
It sometimes takes 2 or 3 tries to get logged in.  There isn't  
anything in the logs to suggest what's going on here.  Can anyone  
suggest a way to troubleshoot this?