The problem sounds familiar to me (unfortunately). I have had the same
problem. Shifting is ok, and the gearbox works when shifting
forward, but when shifting backwards (the lever is shifting though) nothing
happens exept an increase in RPM's.
I'm fairly sure you have a problem with the transmission discs inside the gearbox.
It was happening with me, completely out of the bleu, when i really needed
I was very surprised to found the forward gear working als usual.
In my case i had to replace (by a gearbox specialist) all the wheels/discs/sprockets inside of the gearbox.
I hope you are more lucky