I changed the filament setup of my M300. PTFE tube is going trough a guide now (that's the new part) and then very straight down to the extruder.... and ever since the burn marks I always had on every print have been gone. Guess the reason was filament dust from abrasion which has now (even if it's pretty bended over the guide) no impact anymore.
Strange... or perhaps it's just a coincidence but thought I have to let you know.
Greetings
John