The problem is that the "mail" commands in /etc/fail2ban/action.d/mail*.conf have a wrong "-E 'set escape'" option (it wasn't there before).
According to "man mail"

-E      Don't send messages with an empty body.


so, since -E doesn't take any option, it interprets "set" and "escape" as recipients.
I removed the "-E 'set escape'" from the command.
Maybe that was introduced as a fix for the recent vulnerability in mail-whois-lines.conf?
If so it's not going to work.

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