¿haz intentado una conexión por ODBC con el "Client Access ODBC Driver" (iSeries) que publica IBM?
La idea es hacer una conexión básica en un programa aparte para descartar causas. Aunque evidentemente el dbExpress debe tener algún conflicto con ese Driver.
Tal vez debas revisar actualizaciones del Driver para XE3, en:
http://www.sawatzki.de/download.htm
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The following stuff was written by Peter Sawatzki and may be downloaded: dbxas400 3.0.20a a.k.a. dbexpca400 3.0.20a - DBX 2.5 and DBX 3 driver for Delphi XE3, Delphi XE2, Delphi XE, Delphi 2010, Delphi 2009, Delphi 2007, BDS2006, BDS2005, Delphi7 and Delphi6
latest release 3.0.20a of dbxas400 11.02.2013
added Delphi XE2/XE3 support
Delphi 2005, Delphi 8, Delphi 7, Delphi 6 / C++ Builder 6, C#Builder: IBM iSeries Access driver for dbExpress and BDP .NET driver for iSeries
latest release 2.1.16 of dbexpca400 25.4.2005
fixed invalid cast bug in Data Explorer
fixed CHARACTER FOR BIT DATA() bug
fixed object reference bug in Data Explorer
added REUSESTMT option (preliminary)
fixed bug in parameters handling in PS.Data.iSeries
Source for the driver now available on request to
peter@sawatzki.de.
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un saludo,