Ahh MiRai, much to learn yet!

Many europeans make @ by pressing AltGr+2. AltGr is also known as Ctrl+Alt, so AltGr+2 is the same thing as Ctrl+Alt+2.
Ctrl+Alt+2 may be assigned by default to activate Slot 2 in your Character Set, if ISBoxer didn't detect your AltGr key. (This Hotkey isn't deactivated by Shift+Alt+M.) To see if this is the problem, select your Character Set in the top left pane in ISBoxer, then in the bottom left pane under Slots select the Slot numbers and adjust the "Hotkey to switch to this character". After changing those, Export to Inner Space and you might also want to re-launch your team if it's already up.
If that's NOT the issue, I may need to look for a bug specific to your keyboard input language as selected in Windows. So if you still have trouble, you'll need to visit the region/language control panel in Windows and find out what the keyboard input language is set to, specifically.