Views content pane not showing in panel for some users
The issue that I was having had to do with the fact that I was using Views PHP to enter PHP code as the contextual filter validator. It should have been on only one display, but it was saved in the master, and was added to all displays, including ones which were over-ridden. I removed it entirely, from the master, but it still didn't fix it. It appears that there is a bug in either the views php module, or the view module (or possibly ctools), in which the validation stayed in the views cache, even after removing it, saving the view, and clearing cache.

To fix this, I exported the view, manually removed the validation code, and re-imported.

Hope that helps the next person who might run into this very obscure problem.