SyntaxError: JSON Parse error: Unrecognized token

SyntaxError: JSON Parse error: Unrecognized token '<'
OK
<b>Notice</b>: Undefined variable: json in <b>/host..../public_html/admin/controller/sale/order.php</b> on line <b>2490</b>

Does anyone have an Idea?
I am having this message when I am trying to update an order. Also, for some reason, I am unable to delete old orders.
I am not sure if the two errors are related...

Frais wrote:SyntaxError: JSON Parse error: Unrecognized token '<'
OK
<b>Notice</b>: Undefined variable: json in <b>/host..../public_html/admin/controller/sale/order.php</b> on line <b>2490</b>

Does anyone have an Idea?
I am having this message when I am trying to update an order. Also, for some reason, I am unable to delete old orders.
I am not sure if the two errors are related...

https://github.com/opencart/opencart/issues/3028