This is a known issue right now, that was caused by the update to v1.92 for Sins of a Solar Empire: Rebellion.
We'll be making an update to resolve the problem shortly, but for now you can roll back your Sins to v1.91 by following these instructions:
1.) On Steam go to your games library, and locate Sins of a Solar Empire: Rebellion.
2.) Right click SoaSE:R, and select properties. This will open up a properties/settings menu for the game.
3.) Select the Betas tab from the properties menu.
4.) Select the beta release for "v191mods - v1.91 for Mod Use Only" in the drop down menu.
5.) Close the properties menu, and launch Sins as usual. The mod should now work as normal, and menus will have been fixed.
We will be providing a v1.92 compatibility patch in the near future, with some additional changes to the build regarding balance - so be sure to grab that on ModDB when we release it! Just be sure to follow these steps listed above again and opt back into v1.92 again so that you can play our next patch without issue.