This Mac can't connect to iCloud because of a problem with...

Today, I get a pop message informing me of an iCloud issue:

"This Mac can't connect to iCloud because of a problem with <my email address>

Open iCloud preferences to fix this problem"

But everything looks and works fine. I use all aspects of iCloud except email (I use a non-Apple email address for iCloud/AppleID/iTunes). Every one of them seems to operate perfectly. I have tried logging out of iCloud & resetting everything. I have tried unchecking and re-checking each element of iCloud. But no matter what I try, I end up getting the message above daily (usually more than once).

There doesn't seem to be any rhyme or reason to when the message will pop up. There's no pattern like "it does it when I close Safari" or "it does it when I edit a contact". The only pattern that I can come up with is that it seems to happen only when I am using the computer. So if/when I leave it turned on over-night (and I have sleep disabled), the message does not pop up.

It's just on my MBP running Yosemite that I see this message.