Why can't I access option groups?
OK, I ended up getting into a rabbit hole and spending more time than expected here!

There's a short workaround for you, if you're not trying to edit CiviCase-related option groups:  Comment out CRM/Core/Page/Basic.php line 294, which reads:

$caseCount = CRM_Case_BAO_Case::caseCount(NULL, FALSE);   


For the in-depth fix, I've opened a new bug on issues.civicrm.org here.