Did you stop the sync there? you may need to tell it to continue, even though it is supposed to continue on its own after x seconds. That notice is in the beginning of the sync, not when it is done.
Make a small change to your online tree and sync. Follow that change and see that it took.
Then make a small change to your FTM tree [like changing a name, something obvious] and sync. Make sure the sync completes, you should get a message trees synced successfully.
Let us know. I do sync and my link has never broken. I have had to restore from backup three times and still no broken sync.
A few things: Never use FILE/Close. Always use File/Exit, or the red x at the top right, or double click another tree that you want to view. Why this should make a difference, I do not know, as it shouldn't, but it does [or did].
Never cancel a sync, always stop the process with task Manager.
If your online tree says it is syncing and you know it is not [e.g. FTM says it is up to date] make a small change in FTM and sync.
Those are my collected hints so far.
Let us know how it goes.