Troublesome (13)
(Other definitions for problematical that I've seen before include "Difficult; doubtful" , "Presenting difficulties" , "Open to doubt or debate" , "Dubious" .)
'troublesome' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for vexatious that I've seen before include "Troublesome" , "Annoying" , "Harassing" .)