suddenly I can’t use shift+right click multiple selection. (used it all the time until now)
Does work with shift+left click.
Did you change the select to left mouse button on purpose? Does the regular select still work with right mouse button? If the answer to the latter is no, maybe it’s because you have switched select to left mouse button. To change it back, pull down the user preferences window, click View & Controls, and toggle it to right near the center.
If not, I don’t have a clue.