tested: mails because of starting, stopping, banning.

With the update were 6 files in /etc/fail2ban/action.d changed (may be more, I
just found these):
complain.conf.org  
mail-buffered.conf.org     
mail-whois.conf.org
dshield.conf.org   
mail-whois-lines.conf.org  
mail.conf.org

The difference of the contents in comparison to the files of the former version
are all the same. Please look at every mail command, here for example under
actionstart in mail-buffered.conf:

Old (and sending mails):
Fail2Ban"|mail -s "[Fail2Ban] <name>: started on <fq-hostname>" <dest>

New (I could it never get to send mails to the recipient):
Fail2Ban"|mail -E 'set escape' -s "[Fail2Ban] <name>: started on <fq-hostname>"
<dest>

For testing that I copied these 6 older conf-Files into the running actual
version, restart fail2ban, and mailing is fine again.