Migration issue: Drupal markup isn't rendered on CiviCRM pages
Ah, I think I've seen this before, did you by chance mess with your CiviCRM settings file?

Double-check your CIVICRM_UF setting, Civi might be tripping up if it thinks the CMS is something other than what it actually is.

For Drupal 6 you want CIVICRM_UF = 'Drupal6'