Since this latest update to v28.20 if I click on Navigation and then select “Courses” from the list, the device displays a message “Saving diagnostics” and immediately restarts.
I can only access and ride the latest course added which is visible on the device Home Screen.
It may be coincidental but I deleted a number of courses with the Garmin Connect app and syncing the device prior to noticing this error. However it could just the software version as my device has just updated to this.