by Ken Task.
So how did you try to upgrade from 2.7dev? Download/FTP back up to server replacing moodle code (not overwriting)? Git via command line?
Did permissions/ownership on config.php file change when copying? ie, does config.php belong now to root and readable only by root user?
Compare what's in config.php with that of config-dist.php in the newer version of 2.7+. Are there any differences in lines?
Other than those things already suggested, *IF* you've not developed a lot and site is kinda a tinker thang, you might have to wipe it all out (it was a .dev) and install fresh/new. Unless someone else has a better idea!
'spirit of sharing', Ken