why serial write freezes in Matlab R2012b?

I just installed 32-bit Matlab R2012b on 32-bit Windows XP, but found that the serial write (both fprintf and fwrite) doesn't work anymore. It simply freezes Matlab without any error message. It won't help to change Terminator, DTR, format and mode etc. It seems the problem is related to R2012b, since Matlab R2010b on the same computer for the same USB-serial port (COM4) works fine. Both Matlab versions work fine with built-in serial port (COM1). Any idea? Thanks. -Xiangrui