Clicking on the Link,
https://excellent-appguard.appspot.com/ works for me. I see the Header of the Application "Content About RoboSystems goes here". So, I cannot reproduce the error being reported. However, if it is an issue that sparingly occurs, I recommend that you review the
Stackdriver logs of the App Engine Version to determine what exactly is causing the error.
"Error: Server Error. The server encountered an error and could not complete your request. Please try again in 30 seconds." is just a User Interface error, which cannot be used to determine the root cause of the error.
If you can get the specific backend error in the Logs, you can post the detail here for better advise on the issue.