This article describes two ways of how to resolve DDC connection to SQL database failure

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Instructions

Solution 1

Check transcation log of database and shrink it if it is full. Make sure that the SQL server is running in Primary synchronized, Mirror restoring mode before shrink operation.

Remove mirror from the primary SQL Server
Modify the recovery model to simple
Right click the database and shrink the transaction log
Backup the database and transcation log, copy them to Mirror database server 
Restore the backup with NORECOVERNY option on Mirror database server 
Select the database and select mirror task on primary database server 
Click Configure Security to start Mirroring wizard 
Select Yes on Witness server page

Solution 2
Restore DDC's SID in database to AD

Check the DDC SID in database with that in AD. If the SID of any of the DDC is mismatched in database,we need to restore the SID in database to AD with a tool called ADRestore.

Run .psgetsid.exe Domain\ddcname
Check analystic.service in database
Run ldp in AD to find the objectguid and objectsid of the deleted items 
Run .\ADRestore -r <object ddc> and select the right SID to restore

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https://technet.microsoft.com/en-us/sysinternals/adrestore.aspx