I checked the cache-related headers of the reported website and I was unable to find any issues with them. The headers are correctly returned along with the required values. Also, I was unable to replicate the issue in my test environment.
The errors in question are usually caused by plugins, so you can try disabling the plugins one by one in order to confirm that. If the issue still persists, please open a new ticket from your SiteGround User Area so we can investigate the case further.