Hello JanWalldorf.
A revision 1 for
Gimp 2.6.12 is available. It contains new binaries from the publisher.
After further testing, we also had problems with the Gimp startup in its first 2.6.12 release (on Windows XP SP3).
With new binaries (GIMP 2.6.12 rev1) this problem has disappeared.
Does it works for you too ?