When you are ready to Efile, go to the EF center and highlight the extension to be filed.
Select Efile in the very top header, then Electronic Filing>Convert Transmit Returns/Extensions.
A window will pop up, select "Convert and Transmit selected returns", then UNCHECK "enable conversion error checking", and click OK.
That should file it.

Shame on ProSeries.  This happened a few years ago too.  Had to call a few times to find someone who could give me a solution.

Good Luck.