For one of our custom wikis we have a Special Page that creates new pages based on form input. This used to work in MW1.23 but in the long overdue update to MW1.31 it stopped working.
I am not familiar with Special:StrainNewPage, I am guessing that is custom code not available from a standard extension? If it is not private and you could share that custom code (or at least part of it) maybe someone can help you better to see what is the problem in your case. :-)