Config file is one thing ... version.php files are another.
If linux, logged on as user that can view files in your Moodle code directory, issue
fgrep release version.php
That will show something like:
$release = '2.3.6+ (Build: 20130411)'; // Human-friendly version name
In a 2.3.6+ version of Moodle there are several version.php files
Issue the following at same location:
find ./* -name version.php
That will list all subdirectories that contain a version.php file.
One example in moodlecode/user/profile/field/checkbox/:
fgrep version ./user/profile/field/checkbox/version.php
$plugin->version = 2012061700; // The current plugin version (Date: YYYYMMDDXX)
$plugin->requires = 2012061700; // Requires this Moodle version
So the error you are getting might mean just what it says ... mixed versions ... or files from old versions of moodle still remaining in the 2.3.6+ directory of folders/files.
'spirit of sharing', Ken