Thanks Max,
I experienced the same with the ESP32-CAM-MB - module. Programming worked like a charm, but serial was simply dead. Same in CoolTerm.
I did not even think of RTS and DTR, but that was it.
RTS and DTR need to both be OFF for proper serial connection.
monitor_rts=0
monitor_dtr=0
is the correct setting in platformio for that module
In CoolTerm that is
Options → Serial Port → initial line state when Port opens → both set to off
Thanks for that perfect answer
Regards
Frank