I haven't tried this, but you did initialize the disk in ProDOS? Since DOS 3.3 doesn't support 3.5" disks, You can't easily INIT a 3.5" disk with DOS boot code. The ProDOS boot sectors are trying to find the ProDOS files. There were some 3rd party utilities for using UniDisk 3.5 with DOS. They would INIT an 800K disk as two DOS volumes, or as two drives, since DOS maximum disk size is 400K.

I have no idea whether if you INITed a 3.5" disk for DOS whether Copy II+ could copy Ultima. That would depend entirely on the copy-protection scheme. I'd say unless you're a pretty good cracker your chances are slim. Maybe there's a cracked version of the game somewhere. Haararrr!