The problem seems to be that the Garmin Express update process is pulling the older version of the software. I updated this morning and thought all was well, but I got the error described above. Thanks to the helpful posts here, I checked and sure enough the 'updated' version was 6.5.xx. I manually pulled the 6.6.xx version and that fixes it. It's frustrating enough that plugins are no longer supported, but when the workaround doesn't work, it forces us to the mobile app and I have found that it isn't as accurate.