maybe I am blind, but none of my search approaches with the combination of the words "UART" "Input" "Problem" "PIC24" etc. lead to an post that helps me out. Neither in the forum search, nor via a search engine. Even after I was sure that the analog part of the pin might cause the trouble, the added word "Analog" points to something useful.
The datasheet says only:
Note 1: If UARTEN = 1, the peripheral inputs and outputs must be configured to an available RPn pin.
SeeSection 10.4 “Peripheral Pin Select (PPS)” for more information.
which I have done. In the PPS Section is stated
10.4.2 AVAILABLE PERIPHERALS(...)Peripheral Pin Select is not available for I2C™ changenotification inputs,
RTCC alarm outputs or peripheralswith analog inputs .
The sentence makes obviously no sense, since the physical pin 11 (AN11/C1INC/RP13/PMRD/REFO/CN13/RB13 ) combines RP13 with AN11.
I had expected to read a hint somewhere.
Even in the Analog Port Pins section the description is unclear, since I am not reading the PORT register because the UART is active.
10.2 Configuring Analog Port Pins
When reading the PORT register, all pins configured as
analog input channels will read as cleared (a low level).
At the end, the thing is running :-)