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400 Bad Request
This error occurs if the user lacks the necessary permissions to edit the issue or to view it, or if the mlcs field is not found in the cloud or not associated with the issue's edit screen.
401 Unauthorized
This status is returned if the token or the email is not valid.
403 Forbidden
This can happen if the user does not have the necessary permissions to edit the issue or to view it.
404 Not Found
Returned if the issue is not found or the user does not have permission to view it.
Connection refused
This error indicates network issues. Please check the connectivity between the On Premises instance and the Cloud instance. The following Confluence page from the Atlassian support covers troubleshooting of network and connectivity issues: https://confluence.atlassian.com/kb/network-and-connectivity-troubleshooting-guide-720405335.html#NetworkandConnectivityTroubleshootingGuide-requesttimeout.