Sebastian,
Pin 5 of U2 is fine with that voltage.
I think you may be mistaken by the role of VR8…notice the schematic. VR8 limits the voltage of the LED on the high side optocoupler, while TP1/TP2 are connected to the high side LDR. So this makes me wonder if the solder jumpers are already soldered when you are making this measurement. If so, unsolder them and re-calibrate the trimpots. Let me know if this was or wasn’t the issue…it’s important for me to rule this out when deciding what we should do next.
-Brach