So in the Settings there is a difference between Multiplayer/Campaign Hotkeys and Blitz Hotkeys. The standard Hotkeys for Control groups (ctrl + 1,2,3,… and 1,2,3…) are assigned to the Multiplayer/Campaign section. Creating control Groups in Blitz is possible with the same Hotkeys, but changing the option to select control Groups in Blitz mode is impossible to set to 1,2,3… EVEN AFTER CHANGING THE ONES FROM Multiplayer/Campaign. It always says “The key has already been assigned to another section.”
pls.
> 2535405643941907;1:
> “The key has already been assigned to another section.”
*action
Same problem, looking for solution.
Same problem 
It is because select card 1 is assigned to “1” in Blitz. If you change that then you can assign it to control group select.
I realize this was a few months ago, but just incase people find it via google.