I got this process to work, but it took a bit of extra effort. I found that there was an older version of FTDIUSBSerialDriver.kext hanging out in a place that wasn't mentioned at all. I suspect it was related to the situation that this system had been upgraded from older version of macOS where the older driver was installed.
I used the very useful utility of Find Any File <http://apps.tempel.org/FindAnyFile> to search for all files that began with "FTDI" to locate wherever the kext might be -- and then made sure that only the most current driver was in place (based on the date of the kext). Make sure you adjust Find Any File to search for files that begin with rather than merely contain FTDI and only pay attention to the items that end with ".kext"
The place where I found the leftover/older version of FTDIUSBSerialDriver.kext was at /Library/StagedExtensions/Library/Extensions/FTDIUSBSerialDriver.kext -- I could tell because the date on the kext was from 2012 rather than showing the date of 5/9/17 as it should show for the newer driver.
I found it necessary to go into the Recovery mode terminal to remove the older version and once I had the older version removed, I started normally and installed the updated driver. After installation and another reboot for good measure, the updated driver is working properly.
Find Any File now shows FTDIUSBSerialDriver.kext at two locations on the boot drive -- both show with the date of 5/9/17 as they should. They are now located at...
/Library/Extensions/FTDIUSBSerialDriver.kext
/Library/StagedExtensions/Library/Extensions/FTDIUSBSerialDriver.kext