A repository on your Bitbucket Server instance, which is set up to mirror a user/team on Bitbucket Cloud, is not keeping synchronised properly any more.
Starting from the 29th of April 2019, version 1.0 of the Bitbucket Cloud REST API will no longer be available. This is the version of the API that older versions of ScriptRunner use for mirroring, so mirrors set up on those versions will cease to work after the 29th. However, upgrading ScriptRunner to version 5.5.4 or later will automatically convert any existing mirrors to use the new API and thus keep them working – newly-created mirrors on 5.5.4 and later will also use the new API.
For more information please see the release notes for ScriptRunner 5.5.4.