QTR-Quadtone RIP

I'm using 2.41 in Windows 2000 Pro and have just started upsizing my 
images in Photoshop 7 to 1440 dpi before printing, rather than have QTR 
do it. This has worked fine on 4x5" test images but I've just tried 
loading an 11x14" image and QTR won't load it. When I select the image, 
nothing happens (e.g. there's no preview, no sign that QTR has loaded 
the image). If I click <Print> I get the error message "Unable to 
create TIFF Bitmap buffer."

Any ideas?