If you are experiencing problems with synchronizing your files, the first thing you should do is to update every device in your account to our most recent version for ONE or Groups. This includes devices that are not directly involved in the sync, and even old devices that are no longer used. It is counterintuitive that an unused device not directly involved in the sync can cause this problem, but our Quality Assurance team has determined through testing that having any device in your account, including a no longer used archive device, is associated with file synchronization and performance problems.
Best practice is to delete any device that is no longer needed. This will make your account's journals less complicated and improve performance. It is however possible to upgrade an unused device if you must retain it for compliance reasons.
Another thing you should do is disable LAN-Sync on every device, again including devices that are not directly involved in the sync.