We had something similar happen and it relates to the bug where Field Maps app intermittently logs you out. If this log out has happened after you start mapping off-line, then when you try syncing, when back in mobile data/WiFi range, you will get a sync error. Easily fixed. Flick out the app - or restart device. It will then ask you to log into the app. Then you can happily sync your edits.
I wish esri developers would spend more time testing the app in real word conditions- eg. working offline, and test it on-line multiple times daily, to see when the intermittent log out bug occurs.