A Fork is the same as a collision?
No.
There are multiple types of forks. (Soft chain fork, Hard chain fork, code fork ,etc). You need to explain which type of fork you are asking about. None of them have anything to do with collisions.
I'm not sure what type of collision you are asking about. Are you asking about hash collisions or communications collisions? As far as I am aware there have
never been any instances of a hash collision with SHA256 or RIPEMD160.
How many technical problems do you know about, and which one is the most dangerous ( with Bitcoin) ?
Most dangerous? Uneducated users.