Hi,
The error message means, that something is tempering with the request, and it is not returned correctly, or is completely missing.
It can be caused by multiple things:
– Temporary server outage (very likely)
– WordPress configuration reltated issue, such as the site is accessed via http://www.yoursite.com, but the non www version is the correct. Another usual issue is that the site can be accessed via http and https protocols, but only one of them is correct.
– .htaccess file has an incorrect mod_rewrite rule or flag
– another plugin or a custom code is changing the request
Unfortunately in most cases the issue is unrelated to the plugin. I can take a look if you want to, to see what is returned. For that, please add temporary FTP and back-end details to access your server. Thank you!