Hi,
Error 406 is a very rare error message, and is very hard to debug, as it can be caused by both the server and client side – unfortunately it is very unlikely to be related to the plugin. Here are a few suggestions to try first.
Client side
– Switch your browser and also disable all browser addons -> this is the most likely cause, some addon can intercept the communication and change the headers, causing the problem
– Restart your routers and reset your IP address if possible
– Disable any firewalls
– Restart the computer as well
Server side
– Uninstall and delete the plugin completely, then re-install it
– Turn off any .htaccess or nginx.conf redirection configuration rules on the server side or reset them to default -> this is very likely to be the cause
– Check the server error logs – it must contain something about this failure, before the error was triggered