Summary of the problem-solving shown in the french videos that fixes the "Total Gain"-Error:
The problem lies in the variable gain amplifier INSIDE the RF-Section - The big, heavy block on the bottom module.
The OpAmp in question is U105 and the resistor that needs to be changed is R120.
As suggested in the Video, I changed the value from 390 to 470 and the error was gone.
Unfortunately, the video doesn't really show (I don't speak french and used the youtube-subtitles so I cannot say if he mentioned the exact location) where the OpAmp is located and how to access it.
You get access to it by removing the big bottom cover of the RF-Block. The RF-Gasket holds it in place even after the screws have been removed and requires a bit of force to come off. I used by fingernail as a wedge and lifted it in one corner by about a millimeter. Then the whole cover suddenly popped loose. So better have your other hand ready to catch it and prevent it from jumping up and falling back down on the PCB. Luckily mine took no damage.
The resistor is in the rear area with the green soldermask and can easily be replaced with a 0805 resistor.
A quick check of the analyzer afterwards revealed no immediate change in amplitude of the cal-signal - it was still about -9,7dBm, but to be sure, the analyzer should be properly calibrated after this procedure.
All in all, a rather easy repair, even though it requires disturbing the sacred RF-Block. But a nice prayer to the omnissiah to please the machine spirit and nothing should go wrong.