I had the same issue. I was installing ISPConfig 3 on RHEL 6.1 using the instructions from:
I had to modify it a bit for RHEL though. But everything went smoothly once I went to the RHN website and gave myself access to the repositories.... that are freely available in CentOS without jumping through barely documented hoops (really, what AM I paying for???). If any1 is interested, it was the server-optional repository.
Once I did that, phpmyadmin installed just fine and tested fine. Once I finished the ISPConfig 3 install instructions, it wouldn't work. I appreciate you Jirk for posting your issue and the solution. Everything is fine now here.