3654830 - Error 30252 when connecting to Direct Connect service - MFC DC

Error when trying to connect to a Direct Connect service for DB2 ISQL.
 
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isql -Uxxxxxx -Pyyyyyy -Sxxxxxx
Msg 30252, Level 11, State 0:
Server 'DCS':
[VENDORLIB] Vendor Library Error: No error message is available from the host.
<DB2>
Msg 30000, Level 16, State 0:
Server 'DCS':
Login failure. <SMR>
CT-LIBRARY error:
        ct_connect(): protocol specific layer: external error: The attempt to connect to the server failed.
 
 
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DC's log:
 
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isql               Error: 30252 Severity: 11 [VENDORLIB] Vendor Library Error: No error message is available from the host. <DB2>
isql               Preceding client message generated from host transaction: SystemHandle Tds Object  Error: 0  Severity: 0
isql               Host Message Text: No error message is available from the host.
NULL               NULL    Error: 30000 Severity: 16 Login failure. <SMR>
isql               ...Connection refused by service library 'DB2'.

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Mainframe Connect DirectConnect (MFC DC) 15.0

MFC, Mainframe Connect, DC, DirectConnect, Direct Connect, 30252, No error message is available from the host, 30000, login failure, Preceding client message generated from host transaction, SystemHandle, db2.cfg, comment , KBA , BC-SYB-MFC , Mainframe Connect (MFC) , Problem