Thanks for the quick response Kevin. Yes that was the problem and you lead me to the solution. I only had the roles:
Post Reader
Private Post Reader
Post Contributor
Post Author
Post Editor
Category Manager
and I was missing:
Event Reader
Private Event Reader
Event Contributor
Event Author
Event Editor
The Events checkbox did actually show up under roles > Options > Realm the Post Type Usage. I had looked at that Problems link before but thought it didn't apply since I was already seeing the Events checkbox at that point. But the solution was there - I needed to specify a priority of 1 when registering the Events custom post type. Once I did that, those 5 roles related to the Event type showed up, and checking one of those resolved my permissions issue.
Thanks for your help! Your role scoper plugin adds a lot of power to WordPress - great work!
Aidan