Wow! the more I read the more I think and... You know, I was wondering

about your initial title.  Guess what? it struck me that somebody could think that

you had an "abstract doubt" as opposed to a "concrete doubt" or a real one.

   So, it would be more difficult clear up some abstract doubt, he he.

   When we talk about more serious matters, such as titles of books or studies we use the preposition ON, hence my ''doubts on an abstract".   It seems better, at least to me.

    Let´s wait some feedback on this.

   PP.