Invalid Target' bug

I have been having this bug happen for several weeks now, it first happened on a Lich King attempt about 3 - 4 weeks back and the latest occurence was today at about 00:30 ST.

What happens with this bug is that when trying to use target based attacks (Devastate, Shield Slam, etc) the game will display 'Invalid Target'; as you may imagine this causes no end of problems especially in a raiding environment.

I tried to fix this bug by logging out and restarting all my addons then logging back in but it didn't work and continued to display the 'Invalid Target' text, I then tried turning off addons, logging in, logging out, turning them on again and logging in again, This also did not work. I then tried turning them all off completely but to no avail. I did of course try mutliple characters but all of them had this problem so it isn't just a single character problem.

After giving up I shut down the game and a little later logged back on and found all my abilities working perfectly. (First time I had this problem restarting didn't help me)

Addons:

Bejewelled
PallyPower
Deadly Boss Mods
Postal
Bagnon
Tidyplates
X-perl
Omen

If this helps I also played in pseudo-full screen. (Windowed Mode + Maximized)