We've just had a multiplayer race with a mixed GTR3/ADAC14/ADAC15 field and the ADAC14 ruleset. During the pitstop the GTR3 cars could not finish their pitstop. Instead they were moved backwards into the garage and the players hat to push the reset button on their PC because the game was frozen. It would be great if you can look at that issue for the next patch!
We just tested this in the office. The issue is caused by performing Driver Changes with any car class besides the ADAC GT Masters classes. We recommend against using the ADAC rulesets on any server that uses other car classes.