Calendars works much as it always had. But recently, when Calendars is launched, a black triangle surrounding an exclamation point appears immediately to the right of the “+” sign in the menu bar.

AFAIK, there is nothing that would warrant an alert, but I am vexed re what I did to elicit such and how I rid myself of it?

Suggestions most welcome and appreciated.

What happens if you click the triangle? This looks like some sort of warning.

When I click on the triangle, Jon, I am asked to enter my AOL, password, and I do. I have verified the accuracy of that many times, but a following dialog has always indicated the PW & SN are not congruent or some such.

That is until just now. Now, the dialog says: "The connection with server caldav.aol.com has been closed. Try again later." The only way to dismiss that window is to click OK.

I then opened Little Snitch and verified that I am, in fact, allowing the connection. So might that indicate the issue is on AOL's end?

I'm not familiar with AOL's offerings, but do you have an AOL calendar that you are working with remotely via Apple's Calendar?

If so, that might be the cause of the difficulty. Calendar cannot access the AOL calendar, so yes, probably an AOL issue. If you don't have an AOL Calendar, then look at your calendars in Calendar (click the "Calendar" button) [wow, that's a lot of "calendars" in one sentence] and see if one shows up in the list, along with "Holidays", "Birthdays", etc. Delete it if you didn't think you have/need it.

If it is not in the list, then

Bingo! Upon opening my Accounts, I noticed the check box to enable the subordinate accounts. Apparently the preference needed reestablishing because as soon as I toggled the checkbox, the Alert disappeared (as well as an AOL header that appeared when opening Calendars).

So for now, all is again right with the world, even tough I haven’t a clue re how that AOL IMAP calendar got created.

Of course, Ira, my many thanks!