The 1.9.600 hotfix release includes a fix to the sync problem found in the previous firmware version. You're now able to sync your sessions successfully with Flow again.
In general we recommend syncing data with the Flow web service or app fairly frequently. By doing this you'll minimize the chance of encountering problems when syncing.
Improvements/fixes in this release:
- Training data not synced correctly to Flow web service in some cases, fixed
- 'Cancelled' shown when M400 synced via FlowSync, fixed
- Activity and calory calculation wrong in Swimming profile, fixed
- Training load and fat % wrong in some cases, fixed
Older M400 release notes are available here.