When I start the game I get a message.
X Rebirth - Error
A crash was detected. Do you want to send a crash report to EGOSOFT GmbH?
Then the game opens anyway and can be played normally.

Unfortunately I don't have X-Rebirth from GoG so I can't really say what's causing it. Do you see a crash report generated anywhere? have you tried sending this to both GoG support and posting on the Egosoft forums?

I've actually had pretty good help from both parties.