Actually, I ended up by forcing an update and it asked me if I wanted to overwrite the lites.txt bitmap file which I did and then it cleared the fault. I guess the file must have become corrupt during the original upgrade. However, joeslick112's comment was useful as I have changed the gamma setting which has also made an improvement. Thanks
After I upgraded to 10.4 all the traffic lights on the roads for miles around are intensely and unrealistically bright. Also the VASI landing lights. Does anyone have any ideas as to what is wrong and, more importantly, how I reduce the brightness which is so distracting? Thanks in anticipation.