Comment 3 for bug 678403

The issue seems to be with a required icon missing from the currently-selected icon theme and mail-notification not having a safe fallback option.

I resolved the issue on my computer by temporarily changing the icon theme to "Humanity-Dark" using the Appearance dialog, at which point i was able to create a new account and edit its properties without problems. After this, mail-notification would run normally, even with the icon theme reverted to the normally used one ("elementary Xubuntu dark" in my case), as long as no attempt to add or edit mailboxes was made.