It's a fresh installation filled with lots of 1.9 courses.
I created a course, added 3 users to it including myself (admin) and after this the system crashed.
I can not get into the moodle site anymore.
Activated logging in config.php.
result:

Debug info: Argument 2 passed to has_capability() must be an instance of context, boolean given, called in /var/www/lms/lib/navigationlib.php on line 1134 and defined
Error code: codingerror
Stack trace:
line 397 of /lib/setuplib.php: coding_exception thrown
line 355 of /lib/accesslib.php: call to default_error_handler()
line 1134 of /lib/navigationlib.php: call to has_capability()
line 3039 of /lib/navigationlib.php: call to global_navigation->initialise()
line 781 of /lib/pagelib.php: call to navbar->has_items()
line 4 of /theme/base/layout/general.php: call to moodle_page->has_navbar()
line 765 of /lib/outputrenderers.php: call to include()
line 712 of /lib/outputrenderers.php: call to core_renderer->render_page_layout()
line 106 of /admin/settings.php: call to core_renderer->header()

Moved the sql file and datafolder from the hosting provider to my local server and reproduced the problem.

Is there perhaps a work around?