Jeff,
When creating a new folder, the form allows you to submit without having selected an account. This results in a 500 error when the form redisplays after submit, and a 500 error when displaying the senders list (probably other places too). The quick fix is to re-list the folders and delete the new mis-configured folder.
The app should require an email account on submission, before writing to the db. Even better, if there's only one email account defined, the form should default to having that account pre-selected when the form is displayed.
Jon