If you have the XML Tools plug-in, you can use the following shortcuts for block commenting/uncommenting:
Ctrl+Alt+Shift+C — comment selection
Ctrl+Alt+Shift+R — uncomment selection.
As for the Ctrl+Shift+K shortcut not working, you should probably report the problem to the Notepad++ developer.
Edit: Looks like the standard Block comment (Ctrl+K), Block uncomment (Ctrl+Shift+K) and Toggle block comment (Ctrl+Q) commands don’t work with XML at all. (I’m using v. 5.5 by the way.)
The Notepad++ comment/uncomment functionality is not really intuitive. Here’s a list of the comment shortcuts and related operations that Notepad++ provides:
Toggle line comment on current line / selection
Create block comment on current line / selection (no toggle!)
Create line comment on current line / selection
Remove line comment from current line / selection
Now, since XML only has block comments and no line comments, the only way to create a comment is to use Ctrl-Shift-Q and there is no way to remove a comment as Notepad++ does not provide a Remove block comment feature.
I had the situation to remote comments from a properties / ini file.
used the reg expression ^.#.$ to search and replace with blank (see here
next steps remote blank lines.
This might help you to remove comment from a text file
uncomment on current line / selection
comment on current line / selection