I am having the same problem. I went on a hike with a group which was incredibly slow. When I entered the actual time and mileage I got the same error. I had to lower the time to finally get it to accept the correct distance.
I think I figured it out, with some clues from Garmin Support. I was getting the error message when all I was trying to do was edit the name of the event. I was using the Edit Activity Summary button and getting the error. When I use the Quick Edit button, everything seems to be fine: name accepted and data intact.