Well ... sticky keys are always off ...
And this alt key "bug" happens on 2 computers.
I never tweaked my system. Settings were set at installation time.
When you press alt key and scrub at the same time, there's no update because the alt keys prevents from scrubbing. The grey tick indicating current time doesn't move. So if my alt key remained pressed ( by sticky keys on or whatever ), I couldn't scrub anymore.
But, after that, if you scrub without pressing alt key, as you would do usually, then time field updates.
1) Maya launch.
2) scrub without alt key pressed : no update
3) scrub with alt key pressed : no update because alt key prevents from scrubbing
4) scrub without alt key pressed (like in step 2)) : update

Thank you for your help Joojaa ... please let me know if you got this "bug" ... it might happen only with maya PLE, although that sounds weird ... I don't think my configuration has something to do with it, as I got this result on 2 different computers using the same maya PLE.