As it's happening at the mail inbox
Code:
EsoUI/Ingame/Mail/MailInbox.lua
you should look for some addons containing the "mail" in their name, like
MailR
Mail Buddy
Wykkyd Mailbox
Disable the addons one by one, and check at which combination the error does not happen anymore.
Sometimes it is only happening if you are using different addons together.
It could even be that any other addon that tries to add a keybind, but got nothing to do with the mail inbox, is causing this.