Panels won't display after update to PHP/MySQL 5.5

My server was upgraded to PHP and MySQL 5.5 last night, and I've been very busy upgrading multiple Drupal sites. I have two D6 sites that use a panel for the front page, but even after updating Drupal core, and all modules, no panels will display, even in preview.

I confirmed that the panels still exist, and have content, but all I get are the Vew and Edit Panel tabs where the panel should be.

I've tried searching, but I get nothing but results which have nothing to do with such a problem.

Any ideas?