About once or twice a month I get a day or two of not being able to connect to the email server or the calendar sync from my desktop.  Outlook just spits out CalDav SYNC errors all day long and my iphone continuously says the zoho password is incorrect and asks for me to enter the correct password and I cannot send or receive email.  If I switch both devices to my iphone hotspot, the connection is reestablished and the errors stop.  There is no problem with an internet connection via my home wifi when this happens, I can still surf the internet and go to zoho.com to get emails and shop, everything except get emails delivered to outlook and get emails delivered to my phone.  The issue resolves it's self after a day or two and everything is back to normal for a while.  This is inconvenient and not good for business, as I miss urgent emails and use up all my hotspot data, which can cost me money.

After searching the forums, this seems to happen to many and I have not seen any solutions to this, or even any potential causes suggested.