Hello NSR friends,

im from Germany and over the last months this forum and the posts helped me a lot to get a MC28 with MC21 engine + electrics back on the streets.
Now I'm facing a problem and don't really know where to start, I try to describe:
I am not able to shift in gears 5 and 6 while driving. Shifting in gears 1-N-2-3-4 works perfectly. When I try to shift from fourth in fifth there's no "clicking" sound and I'm landing in a neutal.
When the bike is standing on a spot, regardless if the engine is running or not, then I can shift up in 5th and 6th gear, so then it works. For me it seems like it only doesn't work when the bike rolls.
Transmission Oil is currently 20W50, going to get it corrected to 10W30.
I filled up 0.8l after rebuild. At the rebuild itself i didn't change anything in clutch or transmission, just replaced mechanical seal and checked some other parts.
Gear shift linkage was my first thought, tried some positions but couldn't get it going.

What do you think could be the problem? Clutch, shift forks, oil?

Thank you in advance for your input.
Kind regards from Lake of Constance, Felix