Upon opening a document, you receive the error message: "Category must be single string".
Reason for the error:
The document contains an embedded view and the sorted category is not text.
In the view that is embedded, add the @Text formula to the sorted column formula, like this: @Text(ReqNo)
On the document, if you are using a single category view, add the @Text formula to the Show Single Category object of the embedded view, like this: @Text(ReqNo