@Lazar-Demin I've made an unforgettable error (a "medical malpractice" ;).
I've disconnected the USB-to-Serial module but forgot to remove the wires from TX0 and RX0.
The forgotten ~20 cm / ~8" long wire on RX0 operated as a perfect antenna.
I really do apologise, didn't meant to cause a disturbance in the Force. ;-)