So i finally found out what the problem was:
Our firewall guys updated our CISCO PIX 525 to Firmware 8.0(4) and ASDN 6.1(3) and enabled the SQLnet "Fixup" in the software. That caused the "long" sql-queries to fail. After disabling the SQLnet Fixup in the firewall all is running smooth again...
This is also generated in a the \udump trace file in the moment is execute the query that would have a large resultset:
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Dump file \udump\_ora_4148.trc
Fri Aug 22 09:12:18 2008
ORACLE V10.1.0.5.0 - Production vsnsta=0
vsnsql=13 vsnxtr=3
Oracle Database 10g...
So far we had Oracle DB 10.1.0.5 Patch 2 on W2k3 and XP-Clients using Instant Client 10.1.0.5 an PL/SQL Developer 5.16 or 6.05 to query your DB. This scenario worked well.
Now we upgraded to ORACLE 10G 10.1.0.5 PATCH 25 and now our PL/SQL Developer 5.16 or 6.05 (on IC 10.1.0.5) can logon the db...