First, if you are a OnStepX on a Fysetc E4 user, for the love of all that's holy, disable DEBUG in the Extended_config.h file. The distributed one has debug incredibly set on VERBOSE on the same serial port as the communication with indi. This gets the driver to crash on multiple occasions: syncs and alignment systematically makes indi crash. This is because OnStepX is writing english sentences on the channel indi uses to communicate... a curious choice for a default!
Second, a praise to Alain for writing such a fine piece of driver. Thanks to its capabilities to write interesting stuff when in debug, I was able to figure out the mount was "talking" in plain english where a protocol was supposed to be. Obviously the driver didn't like that! So I searched for a debug option and disabled it and voilat!