Sync is currently experiencing problems. It will be back shortly. After trip abroad.
I recently had a customer who went to Israel to visit family. While he was there, his google calendar, contacts, and Gmail stopped syncing with his Samsung Galaxy Note 5. He assumed it would get better when he got back to the States, but it didn’t. He spent hours on the phone with Verizon wireless tech support, then with Google support, then in the Verizon wireless store. None of them could offer him a solution. I attempted removing the account and adding it again, clearing the data and cache for Gmail, contacts, calendar, and google play services. None of that changed anything.
Eventually I asked him when it began, and he told me the day it started happening. I looked through his emails from that day on the Gmail website, and found a couple emails from his airline in Israel with attachments that had some text in Hebrew. I deleted those emails, and the sync completed successfully.
If you have trouble with google sync, check your emails for anything with special characters or other languages.