Kibana instances work in a pool to run reports, so that a single instance of Kibana doesn't end up being responsible for serving interactive users and also executing reports async. Because jobs can be claimed by other instances, the encryption key must be set to the same value for all the Kibana instances that are connected to the cluster.
I recommend either setting the encryption key for the cloud instance to be the same and allow that instance to run report jobs, or disable that instance from being able to claim jobs by setting xpack.reporting.queue.pollEnabled: false .