Blender offered up a strange problem the other day. For some reason, the Alt key was behaving as if it was sticking, making it impossible to Alt-select edges.
I finally tracked this down to a keyboard shortcut I had set up for alternating between languages. This is quite simple to do in Ubuntu and offers up all sorts of possibilities, and I had set it to change when pressing both Shift keys simultaneously.
Once that was changed to something else (Windows key and Space bar) everything worked hunky-dory.
In a recent re-install of Ubuntu, I cam across the same problem, but this time the above solution did not work. It was resolved by doing the following:
Use CompizConfig Settings Manager (ccsm – install it if you need to), click through to the “Windows Management” subsection and then click into “Move Window”. In the “Initiate Window Move” option, change the button value from <Alt>Button1 to (for example) <Super>Button1. Hey presto, you can now move windows by holding down the Windows key and Button1 on your mouse, and Alt for loop select will work just dandy!