When accessing the preferences from Apple Mail, if you get a blanks screen when trying to use the Fonts, Signatures, etc tabs, you need to delete a settings folder.
Quit from Mail (don;t just close the window, choose "Quit Mail" from the "Mail" menu), delete this folder: ~/Library/Containers/com.apple.mail, then restart Mail.
Note 1: this means opening the Library folder in your home folder, then opening the Containers folder, then delete the folder "com.apple.mail" which should be there.
Note 2: If you cannot see the "Library" folder, go to the Finder (desktop) app, and choose "Library" from the "Go" menu.
Note 3: If you are concerned about losing preferences, move the com.apple.mail" folder to the desktop, or some other location, instead of deleting it. You can then put it back if you want to (of course, the error will probably also come back!)
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