Verifying your cache integrity makes sure all files are up to date and not broken or from an older version of the game. In case some files need to be replaced Steam will automatically do so when verifying the cache integrity. I recommend trying it. Like I said, if it doesn't solve your problem I don't know how else I can help you. Do take a look at the Known Issues sticky thread by Yarlen additional information.
Go to your Steam Library and right-click on Sins of a Solar Empire: Rebellion, then select Properties. Click the Local Files tab and then Verify Integrity of Game Cache.
