The proxy's HTTPS port has been customized but the data connections have not been updated to match this port, a prefix was added to a virtual proxy but the data connections were not updated to match, or in multi-node the reload node where the monitoring apps are reloaded does not have any proxy set up.
Check every " qrs_* " (pre- June 2017 version) OR "monitor_apps*" (June 2017 and newer versions) connection in QMC -> Data Connections
In versions previous to June 2017 (such as 3.2.5 and older), Data connections affected include the following:
In June 2017 and newer versions, "qrs_*" data connections were renamed to "monitor_apps_rest_*"... these need to be modified instead.
Make sure that the URL used to connect to Qlik Rest Connector can be resolved for the node it is reloaded on.
It is recommended to use either localhost (if reloaded on same machine) or the central node FQDN. Using the FQDN in every data connection is preferred.
Note that by default Operation Monitor and License Monitor are only reloaded on the central node.