discuss.answer
OK, "found" the problem. my hoster was running PHP 5.2.17 (I thought excluded is just 5.2.0). After hours of playing around he gave me PHP 5.3.2 and voila: MODx is up and running - did less tests till now but the manager is loading as expected.
Nevertheless thanks to all!!
Take a close look at the 'MODX_CONNECTORS_PATH' section of the core/config/config.inc.php file.
Odd. The error messages you got definitely suggest that your connectors path was missing or incorrect. Maybe your host did something else that helped out with that.