Unknown error when trying to set up Firefox Sync

I try to set up Firefox Sync but as I fill in my e-mail address on the first line i first get the message "invalide e-mail account" (or similar) but then when I continue to complete it with it just says "unknown error". I have tried several different e-mail addresses on two different computers but the same annoying unknown error message keeps coming up. What am I supposed to do about that?

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