ORA-12545 after domain search suffix extension in Windows 7 client

At a large customers' site the IT administration added another domain to the search suffix list in the Windows clients (Windows 7 in this case, not sure wherger 32 or 64 or both).
Anyway since then our application which is using a Gupta (Unify) oragtwy.dll passes through an ORA-12545 error that it cannot connect.

Connection via SQLPlus or TNSPING works otherwise.

All I got from the customer is an excerpt from TNSNAMES.ORA:

OURDB  =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = rac1.company.de)(PORT = 1530))
    (ADDRESS = (PROTOCOL = TCP)(HOST = rac2.company.de)(PORT = 1530))
    (LOAD_BALANCE = yes)
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = xyzapp)
      (FAILOVER_MODE =