On a windows 10 machine (upgraded from Win 7) some strange behaviour of the taskbar is manifesting itself. The leftmost three icons (Windows, search and task view) do not respond to a left button mouse click. All other quick links on the taskbar work. When the mouse is over on sees tooltips for the search and task view buttons (same as my work PC) and a right mouse button click will bring up a context menu. The mouse left button seems to work normally everywhere else so I don't think it is a hardware problem with the mouse.
Some weeks ago everything behaved normally. I don't know when this behaviour appeared and the PC is only used occasionally.
Any ideas on what is going on and how to bring the normal behaviour back.