[eluser]Unknown[/eluser]
From my experience on this issue, config.php or any files within /application/config has nothing to do with this error.
Instead this error could have been the result of accidental manipulation of $config variable in its scope. I was able to fix this error by reverting the file of /system/core/Common.php to its original version 2.0 state.
You might also need to check if you have created your own $config variable on runtime. =)
Best of luck!