This morning, I logged into the CRM, went into the Settings, and looked at the API key listed there.  It was completely different from the one we've been using for over a year.  The new one is longer in length and contains more types of characters (the old one only had hex digits).

I put the new key in, and it works, but I'm still scratching my head over this.  There's no mention of a change in the release notes.  And a recent example given by a Zoho employee still shows the old hex format. (I'm guessing here, but it looks like they might have switched the encryption from hex to base64 encoding.) 

Did Zoho just change the API key overnight without warning and inadvertently break the integration?

In any event, the key definitely changed, and the integration definitely broke.  It's a shame it's during the holidays here, and many people likely don't even know it yet.  Hopefully the customers have some good error logging (and are checking it), otherwise, they're going to wonder why no leads are coming in.  

Making a sudden change like that without sufficient warning, a change which causes API integrations to fail completely, is an extremely serious failure.

Some answers would be appreciated.