Just in case anyone else gets this uninformative error on their monodroid packaging, I got mine when I included an ImageView in my layout and this highlighted two problems.
1. I got the useless message because I hadn’t created an AndroidManifest.xml file, new projects don’t have them by default, so right-click on the project, go to properties, then Android Manifest, and create one. Fill in some details, the file needs to exist anyway, but you can get away with not having one until you put an ImageView in there.
2. Then when you rebuild the project you’ll get a sensible message out of aresgen.exe, which for me was an incorrectly spelt resource name. My .png file was in the correct directory (Resources/Drawable) but I’d misspelled it by one letter.
Hope this saves someone else 30 minutes of searching around for answers.