Then I noticed an error message in the terminal. It said: SEVERE CLFNM0003E: Error getting outline actions for navigator (and so on).
When I looked for that string, I found one hit - which was a perfect fit for the problem.
The recommended fix was to get rid of bookmark.nsf. I moved it from ~/lotus/nodes/data/bookmark.nsf to a temp directory and restarted Lotus Notes.
Now my mail works again. HOORAY! Thank you, Internet! =)