I think you would have to repeat your error checking process.
Try to de-activate plugins again, check your setup with P3, go through the logs, use your debug-plugin again and so on.
The thread here on wordpress.org is a great ressource for searching the error as you can go over the recommendations again but with the certainty that the error has to be somewhere in this one install.
I know this is tedious but the eliminiation of possible causes is usually the way to go. It makes more sense as soon as the most likely cause becomes more obvious.
