I am also getting a "Category Must Be A Single String" error.
I have the following code in the Show Single Category:
MinutesText is a computed text field of the RTF named Minutes.
the view is embedded in a document containing a field named MLastName.
the view selects the form "MeetingMinutes" and shows the Date and MinutesText fields in two columns.
the code: @Contains(@LowerCase(MinutesText); MLastName)
seems to work just fine when I put it in as a column in a view, along with a "name" instead of the MLastName variable.
I would greatly appreciate any help to get this resolved and working.