Can't add media to posts

I have a site that uses the Simple Links plugin for the press page. I’ve run into a problem where if the plugin is activated, nobody can add new media to posts. The “Add Media” button just does nothing. So to work around this, authors have been deactivating the plugin, publishing their post, and then re-activating the plugin.

I’ve asked at the support page for the Simple Links plugin but according to the author, it’s not his plugin that is causing the problem.

If it’s a problem with the theme, I’m going to be in over my head (not that I’m not already) because the original webmaster for the site, and the designer of the theme, died last year.

Can someone help me track down the issue here? Thanks!