Hey there @ThomasSe! @mitchell.hudson, thank you so much for jumping in here, you’re literally amazing 
I am not a C# developer myself by any means, but I did some digging within the forum and here is what I was able to find:
Based on the error code you are getting, it seems like you are trying to send data to a Long Text column that is not accepted by the platform. It seems to me like this could be caused by the quotation marks, as well as a lack of a “text” declare, although those could be caused by forum formatting:
I would recommend making ssure the value conforms with each column’s data structure. You can find out more about this in the documentation, as the link suggests. Here is the data structure that is provided ed within our documentation:
Long text column
To update the long text column, send a string up to 2000 characters with the key “text”.
Raw JSON - use this in your column values variables:
JSON.stringify({ “text”: “Sample text” });
JSON string - use this in the GraphiQL editor:
“{"text":"Sample text"}”
Let us know if this helps We’re keeping our fingers crossed!
-Alex