I owe you an apology!
Having now, finally, upgraded from XP x64 to Vista x64, I am now experiencing the same 1275 error as you. The error appears at the final stage of installation, just as you click the Finish button. The effect is that virtual COM ports do not work in GpsGate (they report the same error on the GUI panel). The GpsGate output service reports Running OK and will sucessfully connect to a BT GPS as an input.
Windows reports two errors in the System log:
7000: The Franson VSerial service failed to start due to the following error: This driver has been blocked from loading
1060: \SystemRoot\SysWow64\drivers\bizVSerialNT.sys has been blocked from loading due to incompatibility with this system. Please contact your software vendor for a compatible version of the driver.
I have tried this installation several times, with and without Windows Defender enabled, however, the result is always the same. Note that my PC is logged on with the Administrator account. These errors do not occur with either Vista x32 or XP x64, just with Vista x64.
Johan, is this enough info for a fix?