Kees,
Check your profile parameter in transaction RZ10 for icm/server_port_0. It may look like this:
icm/server_port_0=PORT=5$(SAPSYSTEM)00,
PROT=HTTP,TIMEOUT=60,PROCTIMEOUT=600
The ITS uses this setting and can have a timeout set to close after a certain number of seconds. You may need to increase this parameter. See SAP Note 1531892 for more information.
Thanks,
Todd