I have been banging my head for quite a while, trying to figure this one out...
In my research of this issue, someone mentioned that their database had an invalid field?
The only application I have used that utilises the database in such a way was RockPOS.
So, I un-installed it and - bingo! Website back to working again.
Obviously the RockPOS module is the issue, so I will be contacting the developer and asking for my money back.