Upon logging into ATM, it is possible that this error may appear.  25420 - The partner closed the socket before the protocol completed. This could mean the partner is configured for SSL Client Authentication and no client certificate was sent to the partner

 

Example:

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In the ATM folder, there’s a log called ATMERR.LOG.  This log entry will indicate the error also:

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Here is an IBM article on this error: CWBCO1034 SSL Error Code 25406 Occurs On IBM i Access for Windows SSL Connection

 

Resolution

The IBM certificate must be installed on all clients