Radek

What version of Marlin are you using? According to the picture, you are using a version that is more than a year old.

If you set #define TEMP_BED_PIN P0_23_A0 or #define TEMP_BED_PIN P0_25_A2 do you get the same error? MaxTemp Error?

Setup #define TEMP_BED_PIN P0_26_A3 and write me if the MaxTemp Bed Error still appears.

But PD0 and PD2 pins are not RX2 and TX2. It would be more appropriate to use PD2 //RX5 PD3 //TX5 

Did you use the same wiring as the other controllers? 

Did you enable it in the configuration.h file? #define Z2_DRIVER_TYPE TMC2209

> > Not exactly. I have no 100 Ohms before TX/RX (only the 1K resistor I have). Also DIAG pin I have not connected and CLK + PDN_UART are also...

Something is wrong here. Active parameters should not be grayed out 

Have you also tried swapping the RX and TX pins? Sometimes the problem is that the wires to the pins are reversed. Does not connect TX to TX. RX connects...

If it doesn't work for you in UART mode, you can remap E0 as Z2 and use the external controller for the extruder in STANDALONE mode. It's not ideal, but...