Hi and welcome to the forums!
We have identified an issue in certain scenarios with the M key.
In my experience it's usually because another control has 'keyboard focus'. This would typically mean that an input box is being typed into (e.g. the platform 'level' box). It sounds like your situation is different perhaps. Could you elaborate on the line you are clicking on which causes the M key to stop working? Is it in the main window, take-off window, or site plan window? Also, what type of line are you clicking on? This information would help us to catch and fix this bug.
In the meantime I have always found that when the M key doesn't work I have been able to resolve it by a single click in the area I want to measure. This sets the 'keyboard focus' back to the main panel and then the M key should work as normal.
Hope this helps,
Leo
