I have exactly the same problem on 2 sites.
One on a dedicated server.
I put the htaccess code and ask my dev to OFF the mod security… but it was never activated.
So is not this problem.
Another on a client site on a share site on OVH.
Same think, impossible to change anything.
I send 2 other emails to the MailPoet dev team and received any answers.
For a paid plugin, I am disapointing.
We must send those newsletters!
Hope MP will find a solution today.