I have had this happen multiple times over the past year. My last error a couple weeks ago, finally got a response a few days ago from support who suggested un-linking and re-downloading my tree. That was the wrong suggestion, since the latest problem was the addition of several hundred duplicate people to my online tree, which required days to fix manually.
My most recent sync failed, and it wants to add 87 more new people - all duplicates according to the sync report, then the program crashed.
I'm very frustrated as you are too. The sync seemed to work for a while, then fails. I've sent off screen shots and sync reports again to support to see if they will forward them to the developers.
I was working with one person there who was very helpful and he got promoted. Now, I get different people responding every time.
My best suggestion is to very clearly document what you are seeing and send that along with whatever other materials you can gather to help them. I used to test software for a living, so it's frustrating to have to do it as a customer.
You can also run the Sync Error Reporter, which will capture some info for them. It's called SyncErrorReporter.exe and is located in the file where your program installed on your hard drive.
My files are too big to send via email so I have to upload to a cloud site. Mine hasn't been picked up by them either.
Patience is a virtue for sure.