iCal won't open on iMac

Hi,

I've had this problem for a few months now and I've been dealing but I've finally reached my tolerance limit. Basically, iCalendar won't open on my iMac. Everything is updated and when I click on it, I get an error message ~10811. However, when I go to the Calendar app in the Applications folder and open it, I can access the calendar by opening the folder "MacOS" and opening the troubleshooting window. However, as soon as I close it, the same problem starts all over again.

I've contacted Apple Support multiple times. Some background: I am a film student and was trying to store too many 4K video files on my computer. It eventually said something along the lines of having so little memory, the iMac is unable to function properly. So, I bought an external hard drive and moved all the files. Once the computer was back and up to its usual self, I ran into this problem. Apple Support, has been, of course, no help.