Hello CHM,
Have you tried flushing your permalinks? That can be done by following the first portion of this guide: Troubleshooting 404 Page Not Found error
It might be possible this event is corrupted to. What happens if you try to create a duplicate copy of this event, with the same dates/times and so forth? Does the duplicate show up properly everywhere? If so, you might just delete the original. Sometimes a server can have a hiccup while creating a series of recurring events, and when that happens that series becomes corrupted.
Does that all make sense? Were you able to recreate it or did flushing permalinks help? Please let us know.
Cheers!
– Brook