As it says on the tin.

I have tried disabling all my mods, I have tried re-installing the game and the DLC, and whatever I do, I cannot land in Vampire Lord form when Harkon asks me to.
I have had only one character that could do so, ironically a custom race I made. The game is not recognising the CTRL key. I have a very good quality keyboard - a QPAD MK85 mechanical - bought new because it is kind to my arthritic fingers, so I doubt it is that, and my hardware is up to the mark.

Does ANYONE have a fix for this? Is there a way to enter sneak via the console.

Help gratefully appreciated!

try going into systems, and then click controls. it might be a good idea to assign a new control key to sneak, or reassign it