Hi,
If I look at the response from the server for the request made by the javascript when I click on the "add to cart" button, I also see the same message:
i.imgur.com/KOhvJau.png
If I try to add a coupon on the checkout, I get the same error:
i.imgur.com/Sv7SyH7.png
I seems that any action I do which involves a POST HTTP request to the server, I get that error.
As this error message is a generic message provided by the web server it doesn't tell much about what is the problem.
You'll have to look into the PHP error log and/or the web server error log for more information about what is going on. If you're on a shared hosting solution, you usually don't have access to these and you'll have to contact your hosting support for this.
The warning message is not related to your problem. It basically means you have a small error in your translation override file. You can ignore it.