Hey @pb84, I think you’re running into a peculiarity with the “JSON” fields in n8n here.
Some nodes would seem to expect an actual JSON value while others expect a string from the looks of it
For the Notion node specifically, n8n tries to convert a string into JSON, meaning the input data needs to be a string rather than an actual JSON object, otherwise the error you have seen is thrown.
The [Object:... result you are seeing would, however, suggest you are providing a JSON object rather than a string.
So my first suggestion would be to try and stringify your object. So instead of using {{ $node["Convert to List"].json["search"] }}, could you try {{ JSON.stringify($node["Convert to List"].json["search"]) }} and confirm whether this works for you?