I have just rebuilt a bike. 1098 Sf On its first ever outing, downshifting it managed to get stuck into neutral. Physically impossible to move. could bend the rod/rearset before moving the layshaft on the box end got a van mate to bring it home. Pulled the generator side off and exposed the gear selector fork setup. It was stuck. Fixed that and in addition fitted ‘69924492A Fork selector spring overhaul kit’ this worked or I thought it did Rode maybe 500 foot up the road and it’s done almost the exact same thing, although this time stuck into first I managed to free it which allowed me to ride back and around the block what can I do now to fix this again that will stop this from happening?
Had same issue with a 748, it was marginally incorrect alignment of the selector drum and fork on two occasions after replacing a worn selector arm (following advice on line to do it without using the setting tool), with the setting tool it worked perfectly 1st time. I think it needs to be more precise than some realise and if you get it right first time without the tool you assume that is a fool proof way of doing it, for me it wasn't.
https://www.bikerstoolbox.co.uk/acatalog/Ducati_Special_Tools.html Not sure if this might be the tool you are in need of, 6th item down.