Andrew here from Clipper Marine.
It could be low idle problem, you should be idling at around 600-700 rpm. A good way is to disconnect the throttle and gear cables and adjust by hand. it could be as you are putting it into gear it doesn’t have enough tick-over to drive it in gear so simply just adjust the low idle.
Any problems give me a call at Clipper Marine (Bavaria Dealers) 02380 605060