I installed the Moodle block MRBS from https://github.com/davosmith/moodle-mrbs/zipball/MOODLE_20_STABLE

First of all I got an fatal error
Notice: Undefined variable: cookie_path_override in /Library/WebServer/Documents/moodle22/blocks/mrbs/web/config.inc.php on line 199 Fatal error: Cannot access empty property in /Library/WebServer/Documents/moodle22/blocks/mrbs/web/config.inc.php on line 199

This error comes from my site cookie path ... I had to delete the cookie path ... not a great idea because my testserver hosts 7 Moodle versions.

The activated block MRBS results the following debug messages:
context_system::instance(): invalid $id parameter detected, should be 0

line 5542 of /lib/accesslib.php: call to debugging()
line 6813 of /lib/accesslib.php: call to context_system::instance()
line 43 of /blocks/mrbs/block_mrbs.php: call to get_context_instance()
line 280 of /blocks/moodleblock.class.php: call to block_mrbs->get_content()
line 232 of /blocks/moodleblock.class.php: call to block_base->formatted_contents()
line 926 of /lib/blocklib.php: call to block_base->get_content_for_output()
line 978 of /lib/blocklib.php: call to block_manager->create_block_contents()
line 349 of /lib/blocklib.php: call to block_manager->ensure_content_created()
line 8 of /theme/leatherbound/layout/frontpage.php: call to block_manager->region_has_content()
line 685 of /lib/outputrenderers.php: call to include()
line 637 of /lib/outputrenderers.php: call to core_renderer->render_page_layout()
line ? of unknownfile: call to core_renderer->header()
line 1363 of /lib/setuplib.php: call to call_user_func_array()
line 91 of /index.php: call to bootstrap_renderer->__call()
line 91 of /index.php: call to bootstrap_renderer->header()