Settings no longer accessible

In one of my projects I can no longer access the Settings page in the Administration section. I get the following exception:

Could not find the specified key "Newsletters" or class id "NewslettersResources".

Hello Michiel van Oosterhout,

Did you, by any chance, try removing the Newsletters module? Such and error might occur because NewslettersResources are referenced in the Basic settings sidebar template, so removing the module gives you this error when accessing Administration >> Settings.
You need to register it again now, and restart the application. You do that by putting the following line in your AppData/Sitefinity/Configuration/SystemConfig.config file of your SitefinityWebApp project

I don't think I removed the Newsletter module. But I did rename the assembly of my custom module after it was installed. When I undid the change to my custom module the Newsletter exception disappeared.