Well basically that means either the track or your cars are a different version to what is on the server. The server is right up to date both with the cars and the track, so you have a track or cars that need updating. Steam should be doing this in the background for you, but there has been many occasions when steam has not updated what is should.
Server = cars and track up to date - You = cars or track or both not up to date ...hence error message.
Go to steam workshop and unscribe from the track ..
steamcommunity.com
With that done, go to you
packages folder in rF2, in there you will see a lot of files ending with
.rfmod...check to see if there is a
RD_S397GT3_VIR_v10.rfmod file present, yours may have a slightly different name, basically any file that has VIR in.....delete this file.
Now fire up your rF2 menu ..now cose it back down. Now go back to
steamcommunity.com
and re-subscribe....now fire up rF2 and see if you can get on the server? ..still get the same error message? then it has to be your cars.
Go to your steam inventory
Click on the car you want to Unsubscribe from
Click view workshop item
Now Unsubscribe from the car
Do that for every car in the GT3 Packs
Fire up your rF2 menu
Close rF2 menu down
Now Subscribed again to all the GT3 cars
Now fire up RF2 and it now should force steam to update the GT3 packs
This fix will normally fix 99% of problems, if it does not, than you may have a more serious issue and you would be best advised to post you problem at the S397 support forum, where I am sure you will be given professional advice on how to solve your problem.