PHP upgrade, broken Horde
I had the same experience about a month ago, also solved by commenting-out ioncube. I also wondered why...
...after a few hours of research I found that ioncube is an optimizer and security patch for PHP array constructs.
The PHP upgrade makes ioncube incompatable and breaks scripts using arrays. It broke more than just Horde.
Anyhow, one thread in here suggested upgrading ioncube --- that didn't work either. I also had trouble installing Zend Optimizer, which also broke Arrays after running for an hour. Go figure?
I've had ioncube commented-out for over a month now, with no troubles.
If you search this forum for "ioncube" you'll find allot of good info about this issue.