This could be a great deal of things. I would recommend opening up a support case in your company's private support area with Jive (You'll find the widget to navigate there in the top left hand of this space)
Some of the things I would do to triage the problem though would be
-Leveraging a UAT enviroment where I can turn the theme off in the community and see if I get the same result
-Look to the admin console logs which output a more verbose version of any error message.
This will help the support team to a quick resolution.