Problem with plugin

Am unable to duplicate, delete, save, or send.

Duplicate, delete leads to a 404 page not found.

Am seeing an error which looks like this when trying to send a newsletter to myself.

Request error not JSON <Doctype

I have checked the memory issue, and that is all fine.

Am using the latest build of Mailpoet namely 2.7.1

Help

https://wordpress.org/plugins/wysija-newsletters/