Using an install of MW version 1.21.1 I found that other extensions were blocked from using preload= on the url bar. I disabled all extensions, and created a manual url preloading a page. It worked fine. One by one I reloaded the extensions (so only was active at any given time) and found that this extension rendered preload non-functional even from a manually crafted URL.
This renders other extensions such as Extension:Inputbox (packaged by default with 1.20 onward) Extension:CreateBox fully or partially inoperable.
Just wanted to post a heads up and ask for a prompt fix as I need all three (the two non-functional plus this one.)