Posted the pic above in the German CRT forum and the pros there let me know that the CRT is definitely getting a 31Khz signal that it's trying to synch to (which of course is not working).
Went back to the J-Pac and installed the second jumper on the 31KHz connector (the one on 15KHz was already installed), now I see that there is a "Synch OK" LED since it is now lighting up green (before only the "Synch IN" LED was lighting up orange).
Is it safe to turn the CRT back on in this state? It's confusing to me that the J-Pac seems to have passed on the 31KHz signal as posted above even though i only had the 15KHz jumper installed. Makes me worry a bit that the J-Pac will not protect my CRT against wrong frequencies at all.
My best guess is that W7 is not picking the 640x480 15KHz but rather the system mode 640x480 31KHz for the secondary monitor. Think i read something about that problem here in the forum, I'll try to find that thread.
Would it help if i chose a mode that only exists as a custom mode, e.g. 544x242, just as a workaround until I know how to handle the 640x480-problem?
I'm still stuck in the middle of the installation process...