adoshrsh

I found it; it was on the Czech site in a spreadsheet document. The resistor is 1 kΩ (some instructions use 3kΩ). Here are the two important photos:
   
The white wire is transmit data from the inverter to the PC (via the USB to RS-232 adapter). So some USB to RS-232 adapters may be loading the transmit data pin too much, or the resistor may be needed to reduce rise time or introduce a slight delay (I can't see how that helps anything). The Czech web site refers to this document with the following text: "ATTENTION - in some production batches of Axpert VM III, MAX and King converters, the display may be damaged by overheating when trying to flash the firmware. A special cable is required ."
As before, I'm sceptical, but this may help someone.

OK, since you're getting past 0%, you must have the timing correct, or at least good enough.
Something that plagues these removable displays is the opto-couplers. Voltronic use particularly cheap and nasty ones, and they do degrade with time as well.
So you may do well to replace the opto-couplers in the removable display, if you're able to do that. I think that there are pointers in the modification and repair topic.