You only seem to have one event on your calendar and that is in the past … can you try shifting it to a new future date (or creating a new test event in the future) and see if this still occurs? If it does not it may indicate a problem in how your theme handles 404s.
If it does persist, can you next try visiting Settings → Permalinks: you need not change or save anything, just visiting this screen should “flush” the permalinks.
If that still doesn’t solve this can you confirm whether the initial troubleshooting steps you indicated you had followed resolved this or not?
Thanks!