The Synchronet Windows NT Virtual UART/FOSSIL Driver (sbbsexec.dll) is a Synchronet component used (in combination with dosxtrn.exe) for the support of 16-bit external BBS programs (e.g. door games).
SBBSEXEC.DLL sends its status and error messages to the Windows debug output. You can view the Windows debug output with a debug-viewer such as Microsoft's DebugView1).
To increase the verbosity of the SBBSEXEC log output, set Debug=true in the ctrl/sbbsexec.ini file.