I have the same issue...
UPDATE: just figured it out... Max baud rate must be set to 57600 otherwise this is the error you get...
You can also get the latest stc @ http://www.stcisp.com/STC-ISP_update_history.html
. MCU type: IAP15W4K61S4
F/W version: 7.2.5T
Adjusting frequency ...