Hi Daniel, it has been a while, but I believe I concluded that the MCU was executing an illegal opcode (as nestor suggested) as at that time I was writing my own small-C compiler for the STM8S and I noticed that there was a bug in the code generated that would cause the MCU to jump to the wrong location on reset under some conditions.
I would suggest that you write some simple code to show that the symptom does not appear and progressively include more and more of the code you are using. What I did myself was to move a loop in the body of main() until I found a point where the STM8S stopped behaving as expected. If UART is part of the problem the loop toggles an IO line but if UART is working, the loop includes both toggling an io line and echoing anything coming in on the UART, that way I can use a logic analyser (you can get a cheap one on aliexpress) and/or a serial terminal to find out at what point the trouble appears.
Please let me know what you find.
