I am having the same issue, when Autoresponder is installed the entire site returns a 500 error.
I have tried every suggestion I have found in this, and other forums…including the absurd idea that changing the database password would fix it (it doesn’t). The config.php is the same as it’s always been, permissions are correct, Common is the latest, ALL stated prerequisites/dependencies are satisfied:
Ubuntu 14.04.5 LTS
Common plugin 3.7.18+20180528
Autoresponder plugin dated 2018-02-11 (it won’t load so I can’t get the exact version number)
What other information can I provide to help solve this?
I even reduced Autoresponder.php to just the four “use” lines and the declaration portion of the class (even removed the __construct() function) and it still throws a 500 error, so I can’t even point to specific function.