Error
Unable to create new mail channel due to connection duplication. Provided mail connection definition is currently used by another mail channel.

Error appears in the version - server JIRA v6.4.6 + Service Desk 2.5.0

This bug doesn't affect functionalities of migrated, newly created email configuration.
Probably after server restart disappeared JSD email configuration.
When we try to set JSD email again we get this error.
When checking the JIRA Incoming mail configuration - there is nothing "duplicated" visible.