Hi,

1/ It seems that's a general issue from WordPress itself.

You can see a topic about this here: (  valid post id and Json is the same )

https://wordpress.org/support/topic/updating-failed-error-message-the-response-is-not-a-valid-json-response-2/

And solution here like other people said is "Update again the permalink"

So, please try go to Dashboard > Settings > Permalinks > and click update again to re-update your permalink.

If it still doesn't work, please try deactivate all plugins ( except plugins come from the theme ) and try it again.

If it works, let's enable each plugin to check what's plugin caused this issue.

2/ I didn't see this issue on your site: https://thegadgetoffice.com/ when viewing any post. So, I can't know what's problems. But, normally this issue caused by your CSS can't be loads. With CSS can't be loads, it can be caused by 2 issues:

a) From your caching plugin.

b) From your SSL.

Sorry because we can't check what's problems because we don't see this issue appears on your site when viewing it.

Best Regards,

PenciDesign