We had the same problem with a different result. One of the updates to Mailpoet 3 since July 17th, 2018 broke the send.
We got the “Oops! There seems to be an issue with the sending on your website. See our guide to solve this yourself.” at the top of the email list.
Clicking on the link sent us to https://beta.docs.mailpoet.com/article/231-sending-does-not-work
following that through we found that the mailpoet-router was always sending us to the home page.
This appeared to be a problem with the way the URL was being configured.
So we went to mailpoet/setting to preview the Mailpoet page. Same thing happened.
We then checked the website to check our PHP settings. We have the website set to run on PHP 7 but the phpinfo page showed we were running PHP 5.
This pointed us to look at the .htaccess files.
In it, we found two PHP handlers lines.
AddHandler application/x-httpd-php70s .php
AddHandler application/php-56 .php
Removing the second one fixed the problem.
Going through the release notes we found that version 3.8.5 released on 14th August, 2018 had this to say:
Changed: End of support for PHP 5.3 and 5.4. Please upgrade to PHP 7.0 or newer!
While the Mailpoet Team told us this was coming, and the there would be a grace period, it appears they didn’t bother telling us the grace period would be between hitting the “update” button for the plugin, and the plugin finishing its install.
Hopefully, this is the problem you are having and this sorts it.