To resolve the issue I added the following line to the uc4config.xml:
<active_keepalive>2</active_keepalive>
Definition:
<active_keepalive>time frame</active_keepalive>
Time in minutes in which the UserInterface sends messages to the Automation Engine in order to keep the connection.
This parameter does not depend on <tcpip_keepalive>. The setting <tpcip_keepalive> activates and keeps the connection on socket level which means that the time interval of keep-alive packages cannot be influenced. <active_keepalive> addresses the program level.
So now the uc4config.xml looks like:
<paths>
<docu type="hh">../../docu</docu>
<!--docu type="wh">../docu</docu-->
<!--browser type="Mozilla Firefox">/users/uc4/firefox/firefox</browser-->
<logging count="10">../temp/UCDJ_LOG_##.TXT</logging>
<trace count="10" ra="0" tcp="0" xml="0">../temp/UCDJ_TRC_##.TXT</trace>
<tcpip_keepalive>1</tcpip_keepalive>
<active_keepalive>2</active_keepalive>
<tcp_nodelay>1</tcp_nodelay>
<SendBufferSize>1048576</SendBufferSize>
<RecvBufferSize>1048576</RecvBufferSize>
</paths>