Manual gearbox - hard to put in reverse - thump when selecting reverse. Opinions ?

Hi,

6MT 335i N54 here.

Today something new happened with my car: when I tried to put it in reverse when parking (drove normally all day, did not push the car), it was difficult to select the gear but it did work. And whenever I put it in reverse with the engine started and clutch pressed, I hear a "thump" just after the gear is selected (clutch still pressed !) but it goes smoothly now. When I put it in reverse with the engine stopped, it goes smoothly. The other gears work normally with engine started or stopped. Clutch pedal feel has not changed.

What could it be ? From the symptoms, is it safe to conclude that the clutch does not fully disengage when pressing the clutch pedal ? I mean, if the clutch would fully disengage, the gearbox should react with clutch pressed just as it acts with the engine stopped because no parts in the gearbox are moving, right ?

I have a further reinforced Sachs Performance clutch, the pedal is quite stiff, I wonder if the clutch cylinder failed in time because the force needed to push the clutch was too high ? Or maybe the clutch plate itself failed somehow ?

Or maybe the clutch circuit needs bleeding ?

Or maybe it's just normal (I don't think so) ?

Any opinions ?
Thanks !