"Im completly out of ideas and hope someone can help me out "
have no idea which paint system you're using but will try
1) you're too damn critical, the customer is happy, so be it
2) some colors are formulated that way to match the factory look
3) wrong reducer-hardener-clear combo, read the tech specs for the system
4) mixing cross brand materials, DON'T
5) solvent burn, too much material OR too little flashtime
6) old finish absorbed the solvent and is out-gassing, primer or filler still out- gassing
7) the moon is in the seventh house and Jupiter aligns with Mars
8) it is just a car and it IS going to the scrape-yard, no matter what
Have tons of fun with it, then grab a beer.