In an act of clumsiness I pulled off the cables connecting the reset button of my Super Snapshot V5 and the PCB. I asked Twitter and Mastodon for help, but to no avail. The search engine of my choice also did not come up with any helpful link. Luckily information about the Commodore 64 is all over the web.
After taking a look at the expansion port of the C64 and tracing the soldering point which was still connected to one of the cables to line C (Reset), it was likely that what the Super Snapshot used to trigger a reset was line A (GND).
Line A connects to a copper frame all around the PCB which made it easy to spot a soldering point that is connected to the frame and that also seems to be of no use for anything else.
Since I am planning some modifications of the module I decided to replace the original cables with longer ones which are a little less stiff.
The reset button works like a charm again and I will do my research before asking for help next time.