Hi Christoph,
Thanks for the update. By the way, if possible can u please inform what type of custom developed system is it I mean what is the platform on which its build like ABAP or web dynpro or its a portal etc.
Bcos CBTA 3.0 SP2 supports the below area's
SAP GUI, SAP CRM Web Client, ABAP Web Dynpro(SP10), Java Web Dynpro (SP10), SAP NetWeaver Portal (SP10) & SAP GUI for HTML (SP10).
Regards,