Journey Builder error: 'One or more custom activities failed to load'

Reason 1: You have started creating a custom activity and have not fully implemented the proper endpoint into its configuration.

In this instance, complete setup of your custom activity and the error will stop appearing.
 

Reason 2: There is an issue related to the service behind the Journey Builder Salesforce Integration. 

If you don't have any custom activities and are receiving this error, please contact Marketing Cloud Customer Support.
 

Reason 3: SSL Certificate validation within the config.json file.

There may be issues within the certificate chain when Journey Builder attempts to validate the Certificate. Review the certificate through an external SSL validation tool and ensure all parts of the certificate chain are complete.