Not a question, just posted to hopefully help anyone with the same problem.
I updated the maps on my Avtex Tourer Two using Garmin Express on a MAC. Afterwards, I consistently got a startup message “Maps are corrupt and cannot be used”. I reinstalled numerous times but still got the same message. And the message was correct - the maps were corrupt and couldn’t be used.
After a lot of effort, I discovered that the problem was the SD card that I had added to the Avtex. I removed the SD card and formatted it. When I reinstalled the SD card Garmin Express said that there was an update to the maps and on reinstalling, the maps were no longer corrupt..