I have had similar problems bringing up the 'Communications' Menu using the '\' key.
I have a Logitech UK Keyboard and have experienced problems on both the latest Open Beta and non Beta editions.
I use the latest Windows 11 Pro Version.
The only way I have found to fix the problem is to add a keybinding specifically using my keyboard.
I have to do this for EVERY aircraft + the General option.
After I go thru the process of adding a key binding to the Communications option I end up with an entry of 'OEM102' (plus I can leave the original '\' there).
This now works on the few aircraft I have modified.
I have not yet tested 'Switch Dialog' & 'Switch to Main Menu' , both of which have a '\' coded with a modifier key.
I could not find a way of specifiying my particular keyboard to DCS hence my long winded solution.
Hope it helps !!
Trevor