What version of the Subscription addon are you using? You can see the version number in the plugins menu of your site.
Looks like something maybe preventing the loading of that module. Do the following to see if there is a plugin conflict on this site:
Test for Plugin and Theme Conflict Before Posting an Issue or a Bug
Also, check the server’s error_log file. That file will contain more details as to what action is triggering the Error 500. That will be helpful to understand what could be going wrong on this site.