This bug basically happens because an image is missing from the game files. This isn't due to a corrupted download or anything like that; the image itself was never included in the release. Several insignia images are actually missing because the images were never made. We've since replaced most of the insignia images with new versions, but there are still races that don't have an insignia image. We've working on making images to cover these holes and prevent the bug resurfacing in the next update, whenever that may be.
The game is simply looking for an image that isn't there. So as long as you put in an image file with the right filename, the game will try and load it - and therefore won't crash when the race joins you. Here's the current image we have for the El Aurians; right-click save as the file. Save it to your Supremacy/Resources/Images/Insignias folder. You will then be able to member the El Aurians without encountering the crash.
