WOW! This one kinda threw me! So, I had the same issue & I saved the form as source files. Generally, an InfoPath form has a file called "manifest.xsf" that much defines it. THIS form had a different name for the manifest! After I saved as source files and renamed the .xsf file back to manifest and packed the files back up, it now at least runs design checker correctly. However, on saving, it renames the manifest file again, so I'd imagine on reopen you'd end up back in the same state. How was this form created, do you know? Later today I'll dig through the files and see why the manifest keeps getting renamed.