Thanks for your help. I've tried the methods suggested in the link but I still get the same error. All the endpoints on the servers are correct, the logins have...

TITLE: Database Properties

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An error occurred while starting mirroring.

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ADDITIONAL INFORMATION:

Alter failed for Database '**********'. (Microsoft.SqlServer.Smo)

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An exception occurred while executing a Transact-SQL statement or batch. (Microsoft.SqlServer.ConnectionInfo)

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The server network address "TCP://************:5022" can...