How many activities do you already have stored in the device's history?

Garmin devices are known for this rather common (across different devices) firmware bug. When the device's memory becomes almost full (due to a large number of stored historical activities) it is supposed to automatically delete the oldest ones to make room for new recordings. However, in reality it doesn't do it properly, and just fails to complete the new recording when it runs out of memory. For this reason, it is recommended to purge recordings of old activities from the device manually from time to time (assuming you don't need them on the device and/or have synched them out to external storage or service).