So, switched the CMOS battery, but that didn't help. Also checked the waterpump (just in case), it starts up when start is pressed (so "no problem").
Further slo-mo video(s) of the Q-code numbers during boot shows the last fully readable Q-code number to be 18.
Also noticed that the small Q-led "CPU" close to the Q-code panel lights up (red), but none of the other 4 small leds lights up (the others being DRAM, VGA, BOOT). The manual states the following: "Q LED check key components (CPU, DRAM, VGA card, and booting devices) in sequence during motherboard booting process. If an error is found, the corresponding LED remains lit until the problem is solved. This user-friendly design provides an intuitive way to locate the root problem within seconds." However, I am not sure if it "remains lit", as it turns off when the mobo shuts down (and then starts the next restart cycle).
Any ideas on what to check or what the problem might be would be quite welcome.