With HTTPS now enforced on LA County public ArcGIS REST URLs, secured REST services can intermittently fail on Geocortex, even when the HTTPS URL is specified. The token URL must also be specified in the Geocortex REST Manager configuration.
While editing the map service, click Edit Connection Settings > Under Authentication Method, click Token Security (by user) > Enter the Token URL, such as https://arcgis.gis.lacounty.gov/arcgis/tokens/ . The token URL must be https and must have an ending slash. Note that the underlined portions in the Token URL must match the Service URL. So if the service is on https://public.gis.lacounty.gov/public/rest/services, then the token URL must be https://public.gis.lacounty.gov/public/tokens/ (the name is specified twice in each URL). The same is true for arcgis, cache, etc. Make sure that the correct Username and Password are entered for that secured REST service as well. Click Test Connection to confirm correct configuration (Test Connection might succeed even without the Token URL, but intermittent issues may come up later).
Secured REST Service Token URL on Geocortex