Hi again,
So for everyone interested in the topic, after replacing the clutch plates the problem disappeared. Thank you for your help guys.
I don't see that clutch plates are NECESSARILY the cause of the problem here.
Friction discs that are only worn, would not manifest as the problem you describe. A released clutch is a released clutch. They are either driving or... they are not! With these monsters, you would DEFINITELY experience clutch slippage under acceleration with worn friction discs.
Swelled friction material or warped discs may cause the transmission to shift THROUGH neutral and that WAS your complaint. They would also manifest as a loud "clunk" when shifting OUT of neutral. This problem would also make shifting into other gears, while under way, rough; sometimes even causing incomplete engagement (and subsequent popping out of gear) or clashing. An adjustment of the clutch lever free-play would likely correct this IF the discs were within specs.
Please post back and let us know what the discs looked like. A grooved clutch basket can interfere with clutch operation as well. Ditto, the incorrect oil.
Since you DID clear the problem, it's likely when you replaced the clutch discs, you also took time to properly adjust the shift linkage and the clutch and bleed the master. It may have been incorrectly assembled by a previous mechanic. It doesn't hurt to PM the clutch and your discs may have been bad.
One further point. When a hot engine is shut down and the discs cool in a compressed state, they tend to stick when the engine is subsequently started. I like to shift the transmission into a higher gear (2nd or 3rd) and rock the bike with the clutch engaged. This frees up the plates and makes starting less of a strain on the starter and lessens the "clunk" when first shifting into gear.