Same with me, when I upgrade my moodle 2.9 to 3.0 and to 3.4 no error during the upgrade but when I logged in to the admin site, it gives me an error of "Coding error detected, it must be fixed by a programmer: Invalid core subtype "register" in renderer request".
When I turned on my debug mode I got the error message below.
Debug info: register
Error code: codingerror
- line 207 of /lib/outputfactories.php: coding_exception thrown
- line 389 of /lib/outputfactories.php: call to renderer_factory_base->standard_renderer_classnames()
- line 2380 of /lib/outputlib.php: call to theme_overridden_renderer_factory->get_renderer()
- line 864 of /lib/pagelib.php: call to theme_config->get_renderer()
- line 113 of /admin/registration/index.php: call to moodle_page->get_renderer()