Error 500 on calendar sync

I used to be able to subscribe to a calendar from Theranest for our private practice. The calendar sync started to fail, so I deleted and tried to resubscribe. The Calendar app on my Mac would not let me subscribe! I get this error: "There was an unexpected error with the request on subscribed calendars (error 500)." I can find no info on this error.

Other people in my practice on Macs have no problem. I used to have no problem. I'm wondering if there's some hidden cache or plist I need to delete. BTW, I'm also the tech support guy for everyone, and have been using Macs for decades. Embarrassing. Thanks for y