I am playing Austria and I am having Hungary as a vassal for more than 30 years, but whatever I do, I am unable to Annex them. I have all the prerequisites, that the game is asking from me, but whenever I click on annex, Hungary cancels vassalization and I lose 50 points of relation with them.

Is there something else to do, to make Annexation successful?

EDIT: LOL this is ridiculous. As soon as I have finished this post, I told myself I try it one more time, and right after I clicked Annex again, I got message "Hungary are now part of our nation!"