IDoc does not change from state 30 to 03

Hello,

I am trying to send out an IDoc via tRFC Port. The IDoc in some cases is transferred successfully trough the port. In some cases it stays in state 30 on sender side. I tried to post-process the IDoc via transaction BD87 and via report RSEOUT00. Nevertheless the IDoc stays in state 30. I do not receive an error message. The only message that is displayed by a popup window tells that 1 IDoc has been selected.

Btw, I am using IDocs of version 3.0/3.1. The sending system is ECC6.0.

Can anyone help me? Thanks a lot in advance!