Calibre content server error on new kindle

[Newwood]

I'm running Calibre 6.10 on a Mac (I know I'm stuck in the past, but there are reasons...) and love using the Calibre Content Server to transfer books to my 7th generation Kindle.

The problem: My spouse got a new Kindle but when trying to transfer books to it using content server (using same web address as the old kindle http://192.168.0.169:8080/mobile) the Kindle correctly loads and shows the books on the web browser, but when clicking on a mobi title it displays the error:

"Could not load the requested resource. Error code: -370 (net::ERR_INVALID_HTTP_RESPONSE)."

Is there a way around this adjusting the content server settings?

Thanks!
Newwood