I'm experiencing a similar issue. I haven't tested everything, some new content works, but when trying to practice/race with the GTX cars, session launch fails with this error and I end up at the race selection menu. I owned content prior to the sale, but purchased the all content black Friday special.
please contact hannes at the help dest @ http://support.raceroomracingexperience.com/index.php/ and give them a few days to respond.
Getting the same error Trying to submit a ticket, but the website keeps stalling out once I hit submit.