The JSON Parse Error issue where the response contains error 403 Forbidden should be fixed now.
Additionally Plex Media Server beta 1.14.0.5465 has retries for failed requests to fetch the guide - so that in itself would have helped whilst the issue was there
Please do manual update to the guide and once that completes ok, then incremental updates would ensure the guide remains up to date
If problems persist, please provide fresh logs from the server with debug logging enabled beforehand and also a zip of the Diagnostics directory which is within the Plex Media Server data area
https://support.plex.tv/articles/201643703-reporting-issues-with-plex-media-server/
https://support.plex.tv/articles/200250417-plex-media-server-log-files/
https://support.plex.tv/articles/202915258-where-is-the-plex-media-server-data-directory-located/
