for the buffer-size I found this:
"For all serial port VIs, the default buffer sizes are 64 for Mac OS, 1024 for Windows Me/9x, and 4096 for Windows 2000/NT/XP. If you specify a buffer size of 0, the system keeps the same buffer size as before. The maximum buffer size on Mac OS is 32767 bytes, or 32 KB minus 1. The maximum buffer size on Windows is 65534 bytes, or 64 KB minus 2."
when I try the VI and use another non-specified baud rate, I also get the error you discribed...
for the moment I don't know how to get the serial-communication to work with a baud rate out of standard...