Here is Google's Official Shortcut Page for Google Doc. There you find that the current shortcut to insert comments is
Ctrl + Alt + M and the old shortcut was
Ctrl + M. I would have to assume that the old shortcut isn't supported. Which leads to this...
"Originally, US PC keyboards (specifically, the US 101-key PC/AT keyboards) did not have an AltGr key, it being relevant to only non-US markets; they simply had "left" and "right" Alt keys.
As those using such US keyboards increasingly needed the specific functionality of AltGr when typing non-English text, Windows began to allow it to be emulated by pressing the Alt key together with the Control key:
Ctrl + Alt ≈ AltGr -- (wiki source).
Therefore, it is recommended that this combination not be used as a modifier in Windows keyboard shortcuts as, depending on the keyboard layout and configuration, someone trying to type a special character with it may accidentally trigger the shortcut, or the keypresses for the shortcut may be inadvertently interpreted as the user trying to input a special character."
US International Layout:
Ctrl + Alt ≈ AltGr ... which means...
AltGr + M = µ
Note: To resolve this issue switch to a pure US layout, not a US International keyboard layout. See steps below on how to switch.
Windows 7: Switch Keyboard Layout to Pure US Keyboard Layout:
- Open Start Menu
- Type in: Region and Language
- Hit [Enter]
- [Click] the Change keyboards button
- Select the General tab if not selected
- [Click] the Add button
- Scroll to and expand English (United States)
- Expand Keyboard if not already expanded
- [Check] the check box for US
- [Click] the OK button
- Select 'English (United States) - US' from the Default input language drop down menu
- [Click] OK