Please enter the TIMER_SERIAL define in variant.h of rumba32.
If the SoftwareSerial.h library is included, as is the case with the TMCStepper library, SoftwareSerial takes the timer 7 for the STM32F446 processor which is the same one used by the servo and in fact no longer works.
I put #define TIMER_SERIAL 8 in variant.h so the servo works, but I don't know if the SoftwareSerial library works, it should be verified ...