You have installed Microsoft Windows XP SP2, but you have not installed hotfix 919477. If you want to connect to a system with NWBC, the connection fails. An error message appears: Connecting to 'SYS' failed. The backend of connection 'SYS' returned a fatal error: An unknown error occurred while processing the certificate.
Request and install hotfix 919477. For more information, see 7.2 Use of Digital Certificates.
Note that in Microsoft Windows XP SP3 the hotfix is included automatically.
For SAP-internal developers, the hotfix is available at \\NS0009\nwuif\nwbc\download\winhttp_patch\XP.