Heating problems after conversion MK3s+ -> MK3.5s  

Heating problems after conversion MK3s+ -> MK3.5s

Hello everyone,
As the title suggests, I have decided to upgrade my MK3.
In the meantime, however, I almost regret it.

It all started with the deliverer sludging the parcel. Well, the parts have now arrived and I started the conversion the very next day, which went perfectly.
The same cannot be said of the commissioning. First of all, it couldn't find the firmware on the stick, after multiple checks and even fresh FW from Prusa on the stick, it wasn't even found by the PC, UBS stick defective. The other stick worked immediately.

Then it was straight on to the calibration, which went well up to the point of the heating test. Then he stopped the test and shouted that my sensors were probably broken. But they weren't. The heaters simply didn't get warm at all because it didn't switch them on.

After some checking and researching on the xBuddy board I realized that the heaters and many other components on the board have control LEDs, nozzles heater = D13, heating bed = D15. However, these do not light up for me.
I then checked the heaters separately and the temperature sensors too. It's not the parts, the Heatbed is completely new and so is the sensor. The nozzle also works.

Now I really couldn't help myself any more, so I contacted the support team, made videos, took photos, explained my errors what felt like 10 times, was forwarded to other technicians several times and and and...
First I got a new xBuddy board sent to me, installed it immediately after receipt, same problem again. Again support, now it should go to the technology and they would contact me, 2 weeks around, no message, so wrote again to the support, now he said, must be the display, I'm still waiting for it.

Today I sat down and searched online for the wiring diagrams, found them under MK4 here in the forum, so I looked at the xBuddy's wiring diagrams.
I have to say that I am an electrician and also work with circuit boards.

Because I now have 2 boards, I was able to measure and work on both in parallel in my workshop. Now I have seen in the plans that the board has several MOSFET's:
Q6 = FQD11P06TM is for the voltage enable, I measured this and it works when the board boots.
This is controlled by Q5 = MMBF170 which is switched directly by the STM via pin PG10 = Header_Enable. Everything works up to here.
If you now activate a heater in the menu and supply the corresponding mosfet Q3 or Q4 (nozzle | bed) = CSD18503Q5A on pin 4 with +3.3V, the heaters will also work.
However, I have now discovered that there is a 74LVC1G11 = U22 and a 74HC2G08 = U14 in the control line to these MOSFETs.
They obtain their data from several sources. From here, it becomes relatively critical to bridge or measure this somehow.
I'm just wondering why I have this problem with two boards and what can cause it, especially since everything is connected correctly.

Maybe someone else has an idea what it could be or what I could check.

Thank you in advance