Description: I was performing an EE2 to EE3 update on a site and updating all the required add-ons, including Custom System Messages. Upon update, the admin.php page rendered as a blank page. Along with EE support, we were able to trace the error to the following line in CSM.

“The problem is- go look in exp_extensions- these are the methods/hooks for system messages:

40   System_messages_ext   output_show_message   output_show_message   1   3.0.4   y
41   System_messages_ext   extend_output_class   sessions_start     1   3.0.4   y
42   System_messages_ext   template_post_parse   template_post_parse

You’re getting the error because the method extend_output_class doesn’t exist in the file.

BUT in the new file, when it installs- these are the hook/methods:

array(‘hook’=>‘output_show_message’, ‘method’=>‘output_show_message’),
array(‘hook’=>‘sessions_start’, ‘method’=>‘sessions_start’),
array(‘hook’=>‘template_post_parse’, ‘method’=>‘template_post_parse’),
It didn’t update them like it should have.”

Detailed steps to reproduce the issue:
1. Perform Update from EE2 to EE3.5.3
2. Navigate to Control Panel

An uninstall/install of the add-on fixed the issue, but I wanted to advise you about this. Thanks!