For whatever reason your dialog template format variable could out of scope when your call the Dialog function to create the dialog causing the error. Scope problems are usually the result of the scope rules imposed for UDFs. For example, you would need to '#include' the template file in a UDF if you display the dialog while in a UDF.
Another possibility is that you are missing the format variable from the dialog template because of some editing issue or something.
FWIW, you can find formation on when a feature was added to WinBatch by searching the "Fixes and Enhancements.txt" file that is part of each WinBatch distribution.