Many times upgrading older websites from 2.8.x to 3.0.x I often got this error. And every time the problem was in corrupt paths of the config files.
Check the three config.core.php in the root and in /connectors and /manager and the config.inc.php in core/config.
This was my disaster thread: Every upgrade from 2 to 3 ends in a disaster
Maybe it can help.