Undefined index

Hello,

When I turn Wordpress debug to true (in wp-config.php), I see the same Undefined index error from many wpmudev.org plugins:

Notice: Undefined index: page in /var/www/clients/client1/web1/web/wp-content/mu-plugins/blog-activity.php on line 32
Notice: Undefined index: action in /var/www/clients/client1/web1/web/wp-content/mu-plugins/blog-activity.php on line 36
Notice: Undefined index: reset in /var/www/clients/client1/web1/web/wp-content/mu-plugins/cets_default_user_role.php on line 272
Notice: Undefined index: report in /var/www/clients/client1/web1/web/wp-content/mu-plugins/reports/user_comments.php on line 34
Notice: Undefined index: report in /var/www/clients/client1/web1/web/wp-content/mu-plugins/reports/blog_comments.php on line 34
Notice: Undefined index: report in /var/www/clients/client1/web1/web/wp-content/mu-plugins/reports/blog_pages.php on line 34
Notice: Undefined index: report in /var/www/clients/client1/web1/web/wp-content/mu-plugins/reports/blog_posts.php on line 34
Notice: Undefined index: report in /var/www/clients/client1/web1/web/wp-content/mu-plugins/reports/user_posts.php on line 34
Notice: Undefined index: report in /var/www/clients/client1/web1/web/wp-content/mu-plugins/reports/user_pages.php on line 34
Notice: Undefined index: page in /var/www/clients/client1/web1/web/wp-content/mu-plugins/reports.php on line 54
Notice: Undefined index: page in /var/www/clients/client1/web1/web/wp-content/mu-plugins/supporter-ads.php on line 47
Notice: Undefined index: page in /var/www/clients/client1/web1/web/wp-content/mu-plugins/supporter-framework.php on line 52
Notice: Undefined index: key in /var/www/clients/client1/web1/web/wp-content/mu-plugins/tips.php on line 39
Notice: Undefined index: updated in /var/www/clients/client1/web1/web/wp-content/mu-plugins/tips.php on line 44
Notice: Undefined index: activated in /var/www/clients/client1/web1/web/wp-content/mu-plugins/tips.php on line 44
Notice: Undefined index: key in /var/www/clients/client1/web1/web/wp-content/mu-plugins/upgrades-framework.php on line 37
Notice: Undefined index: page in /var/www/clients/client1/web1/web/wp-content/mu-plugins/user-activity.php on line 36
Notice: Undefined index: page in /var/www/clients/client1/web1/web/wp-content/mu-plugins/blogtemplatesfiles/blog_templates.php on line 499

How can I start to troubleshoot this issue? I suspect this is one bug causing the multiple Undefined index errors, and not a series of bugs.