I mean，you can try to make a new project to confirm the function SystemClock_Config can only work at 4.194M? Maybe it's not the frequency of modification.

It's really weird. Can only work at your initial clock frequency 4.194M??

​Are you generating code with stm32cub? Try to check conflict or make new project to test uart alone. I used this chip, if i set clock below 16M, uart can't work well.

You can try to set clock above 16M​.