The key combination is used in Emacs to mark from the point to the end of the line (navigation with shift selection), but a box is overriding that making it unusable.
I have been working in another station with Fedora 24, and the problem doesn't show, so it seems to be derived from Fedora 25 (the version of Emacs is the same in both stations).
Here is a sample of the box (right after the box appears it has the @ symbol):
Alternatively, once the box is showing, if I do ctrl+x then I get:
In order to solve the problem, it is needed to know what the box is, and I think I prove that right, because unsuccessfully spent many hours "googling" for something I don't know how to name or describe, the GUI doesn't show a gloss, apologies for my ignorance.