Hi Ralf,
thanks for the code, looks good. So I'm pretty puzzled now... 
In this case there are only a few options left:
- Are you running the most recent version of SAP GUI (at least 7.30 PL6 or 7.40) - the more recent the better? If in doubt, please update.
- Check transaction SDSH_CONFIG. Is the flag set?
- Check your SAP GUI client settings (Options - Interaction Design - Visualization 2): Enhanced Search must be enabled (...Automatically)
If nothing works, I need to check your implementation directly in the system. Please open an OSS ticket with an open system connection for that. Component: BC-DWB-DIC-F4. Most certainly it will reach me as well. 
Best regards, Sebastian