This started a week or so ago. I had a program pinned to the task bar, but suddenly it was behaving strangely. When I ran one or more instances of the program, they would group together, but not group with the pinned icon. I could move them separately. Just now, in a attempt to fix this, I unpinned the program, and tried to re-pin it, but that option is gone! Dragging the .exe file to the task bar also gives me the 'not allowed' mouse cursor. Pinning any other program and pinning this program to the start menu is still possible.
Before you ask, I've tried the shortcut arrow fix, but I already have, and always had, the IsShortcut key in the registry.
I haven't installed or uninstalled any programs in the mean time (except for updates), and a reboot didn't work either.