Hello Klaus,
In addition to the above parameter rdisp/gui_auto_logout, do check the following things.
1. Make sure that the network connectivity is good, i.e. No Packet Loss.
2. Try disabling Antivirus, especially McAfee.
3. Add(allow) SAP GUI as an exception in Windows Firewall and Antivirus Firewall.
1139596 - SAP GUI: Connection to partner 'sapserver:sapdp00' broken
regards,
pavan