Could not load file or assembly 'System.Data.SQLite' or one of its dependencies. An attempt was made to load a program with an incorrect format.
I wasn't even using SQLite in my project; at least that's what I thought. Turns out ELMAH was trying to load it.
The error is due to trying to load the 32bit SQLite.dll on a 64 bit server.
To fix the error set Enable 32-Bit Applications to True for your Application Pool under Advanced Settings.