Zoho mail disappearing in Outlook set up with Exchange ActiveSync

Zoho mail disappearing in Outlook set up with Exchange ActiveSync

This morning, my Zoho Mail inbox in Outlook 2016 on a Windows 10 PC emptied out. New incoming emails show. I deleted the .ost file. The emails populated, then disappeared. All emails are there when I log into Zoho Mail via a browser.

What's up? Is this a known issue, or my problem only? How can I fix it?