Hi Brandon,
Did you found what caused the issue - I have similar porblem
Ilya
In my case, this is done by one of the plugins (https://moodle.org/plugins/mod_groupselect) using an admin string definition. Definition was removed during upgrade, so it's no longer present in code and moodle is complaining.
Solution to this is edit mod/groupselect/edit.php: change get_string('configrequiremodintro', 'admin') to get_string('configrequiremodintro', 'mod_groupselect')
and paste $string['configrequiremodintro'] = 'Disable this option if you do not want to force users to enter description of each activity.';
to mod/groupselect/lang/en.php
This might be caused by another plugin as well, search your moodle directory folder for 'configrequiremodintro' recursively and you'll see !
Hope this helps
M.