Hello all,
I had the same problem, but it turned out that I made the silly mistake of copying my wp-includes from my backup, instead of the latest wordpress version!
I didn’t overwrite the wp-includes that was already there, but deleted what was on the site, then re-uploaded (like crazyrobot said).
Also, the secret key is not the only missing line in the wp-config file. There are also 2 define statements after the database definitions that may maybe needed to overcome this fatal error.
Hope you all get it working.
