The 9V regulator should be a normal 7809.

Good to know - but who sells those?
Thanks for the tips about the short. I'd find it very surprising if I had a short anywhere as all the solder joints are perfect under the dissecting miroscope, but then again, I've never soldered anything like that, so surprises are to be asumed
I'll try to do the tests you've suggested, although I have to admit that the task looks somewhat daunting to me right now...