JSON error - what is this and what to do to correct it?

I get two JSON errors:
Printscreens images are attached.

In admin area
1. Sales>Orders>add order --> the JSON error pop up when clicking (+) Add order.
2. Sales>gift vouchers> send newsletter --> when clicking the mail image (send) also a JSON error appears-


I suspect that the error is not locate to a single php file.- but to the JSON.I can not locate any errors in php files, I do not know what JSON is, but maybe something that site use to do certainn tasks....!? other - I do not know.

....Or is JSON located to server files/ engines?

First of, what is JSON: https://en.wikipedia.org/wiki/JSON
And this is heavily used by OC - has nothing todo with the server.

Second, could it be that you are using old extensions - not compatible with your shopversion 2.3.0.2 ?
Because the message shows that this function is missing the value for currency.

Thanks for the tip and the education. I will try to remove one install and check if the errors disappers