Just saw this post so I am a little late to the party.
I've recently replaced the Kanzaki-Hurth KBW10 transmission on my yanmar 2QM20. I had similar symptoms as the OP. I decided to replace it with a with a good used unit, exact same model but newer. I have the failed original at home on my bench waiting for winter when I plan to tear into it.
I happen to have the factory manual for the KBW5/10/20. The OP mentions that he has the HBW10 and here is an online manual for that transmission: Boatinfo - Hurth 5 10 20 Service Manual
Briefly looking over that manual, it seems that the HBW is very similar to (if not the same as) the KBW. So that manual may be helpful to owners of the KBW as well.
I want to point out that I believe that the service bulletin referenced does not pertain to the KBW or HBW transmission. The service bulletin applies to:
"mechanical type gearboxes and sail drives only ... Marine gear Models KM2P, KM35P, KM35A and KM4A".
I believe these are the cone clutch style transmission that use motor oil in the transmisison. The KBW/HBW transmissions do not have a cone clutch and use ATF.
Which begs the question: where do the KWB/HWB faithful set the shift lever when sailing? I have always understood that for this style of transmission it was best to place it in reverse. A few mechanics have told me so, but I am starting to wonder.
I hate sailing with the trans "locked" in reverse because of the drag. I know I could squeeze another 2 or 3 knts out of her if I put it in neutral :-)