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
