On July 19, 2018 a critical update from Salesforce will automatically be activated in all Salesforce orgs. This update causes external pages to not work anymore. Because of that, it is important to execute the following steps before that date.
1. Go to the Salesforce Setup
2. In the quick-find bar, type 'Critical Updates' and open it
3. On this screen, look for the 'Secure Topics for Salesforce Sites and Portals' update and activate it
4. After you have activated the critical update, enter Chatter in the Quick Find box in Setup. Select Chatter Settings.
5. In the Topics section select 'Allow topics in Salesforce Sites and Portals'.