Custom Theme 404 Error with Permalinks

I have searched to the edges of the internet for a solution to no avail. I am volunteering for an association, their custom built theme is returning 404 errors when I use any permalink option besides default. So far it’s only effecting the modern tribe events custom posts.

For a while I was able to fix the issue by manually changing the post URL. But that no longer works.

I reached out to modern tribe and they troubleshooted the issue with me. Disabling all plugins doesn’t help, but switching to the twenty fourteen theme fixes it. This led them to believe it was not a plugin issue but a theme issue.

Then I discovered several posts that blame the .htaccess file for this issue. I edited it, adjusted permissions to 644, and eventually deleted it so WP could generate a new one. None of that worked.

Other posts blamed the mod_rewrite on the server. I contacted Bluehost and they spent all morning checking settings etc… They eventually told me that it wasn’t a server issue and was most likely an issue with the theme. Especially since I don’t have the issue with the twenty fourteen theme.

I have no idea what in a theme would cause this. Would it be in the functions.php file? What should I look for? The theme developer was a former member of the association that is long gone so I have no resource for this.