The "Toggle Comment Selection" (Ctrl+/) works perfectly with downloaded standalone Qt Creator 2.8.1 (built with Qt 4.8.4), but does not work with Qt Creator 3.0.0 (built with Qt 5.2.0).

These are both About dialogs from working / not working Qt Creator installations:

Qt Creator 2.8.1
Based on Qt 4.8.4 (GCC 4.6.1, 64 bit)
Built on Aug 23 2013 at 13:39:10
From revision c3ed746c24

Qt Creator 3.0.0
Based on Qt 5.2.0 (GCC 4.6.1, 64 bit)
Built on Dec 10 2013 at 11:45:55
From revision 27d10d8dcd

There is no shortcut collision.

In Qt Creator, the Environment - Keyboard dialog allows me to successfully configure a shortcut with "Ctrl+/", by using the left Control and the slash in the numeric keypad. The key combination is correctly detected and saved, but after applying changes that key combination is not toggling comments in selected code.