I've got a problem using the deflicker option for a series of RAWs I've shot. In that series I've got a serious jump in the exposure. Correctly LRTimelapse recognized the shots at this jump as keyframes. I used Adobe Bridge and Camera Raw to adjust the shots perfectly. Then I reloaded them into LRTimelapse. If I now use the deflicker option and save the xmps, the xmps of the keyframes are changed. As a result the shots at the jump do not fit anymore. Is this supposed to happen? How can I avoid this? Can I force LRTimelapse to leave the xmps of keyframes fixed or is there a workaround?

Yes, there is an option in the settings (expert) that will force LRTimelapse to leave the keyframes alone.
Generally I don't recommend using deflicker on Sequences with lots of holy Grail keyframes - respectively to take care not to produce flicker when shooting with the holy grail technique.