Hi, try to disable security plugins, or if you have the 404 redirection plugin disable it.
Ask the provider to disable the mod_safe module of apache as well.
If one of those operations work, we can try to find why those protection are triggered.
BTW it’s not the first time this happens and every time was a worng detection of “unsecure traffic” by one of those plugins.
Let me know if disabling one of them solve the problem so I can look forward to solve the problem even on Newsletter plugin side.