I found opening outlook in different modes cleared the error.
Its not as academic as registry keys changes etc and probably similar to tapping it with a hammer.
1st open with the restore switch: Windows Key + R and in the dialogue box type, "Outlook /restore" (obviously without the "").
Then close the program.
2nd, open in Safe mode: Windows key + R and in the dialogue box type, "outlook /safe" (obviously without the "").
Then close the program and open in normal mode with the normal shortcut/icon.
It is OK so far, I have only tested it (by opening and closing the program many times for 30 mins), so I am crossing fingers.
Update:
I didn't realise that some others had already suggested /restore switch.
Anyway, its all still working, which is great.