I open this discussion because I was installing the Certificate module in moodle 2.3 and I've found some issues.
The first one is tha there is a new capability that must be added for all the modules. This is called "addinstance" and in the module is not found.
For fixing this is necessary to change the /db/access.php file and add in the end of the array:
'mod/certificate:addinstance' => array(
'captype' => 'read',
'contextlevel' => CONTEXT_MODULE,
'archetypes' => array(
'teacher' => CAP_ALLOW,
'editingteacher' => CAP_ALLOW,
'manager' => CAP_ALLOW
I will take a look on the module behaviour on this week. If I find something else I will post it.