Radek

Do you have a properly oriented serial cable? In the second photo you can not see the color indexes on the cable. Check the conductives of the serial (black) cable.

Send a photo of the cable marking on the side of the display

OK, the wiring looks fine.

Do you have the same serial port communication speed set on the display?

Does the printer respond to the M503 command?

I asked about the response to the M503 command

So that's okay too. Can you try the display with a different printer?

Did you check the cable to see if any wire is broken? The wiring appears to be connected correctly. You say that the serial port speed is set to the...

I'm not from BTT. This is not a HW intervention in the board, so it will not affect the warranty. Could be a faulty pin on the SKR or TFT....

If you want an answer from BTT, go to their website. BTT support is not responding on github.