Thanks for your tip.
I've contacted them but they don't support AA anymore and also don't have any schematics.
But i've looked into the schematic I found on the Internet and the problem is a very very very and VERY unstable +/-15V stabilizer circuit
It looks like this section was designed by a nono because you have luck when this circuit is working at starting up the system.
I’ve made some fast modifications and now the +/-15V section starts up AND is stable
I'm thinking about chaning tis section complete by inserting a shunt-regulator.