PrintMonitor error messages can actually be decoded if you have a list of Macintosh system errors. The "why" (or y=) portion is the system error number.
Why=-120 means "Directory Not Found". Why=-120 may be the result of a corrupt directory structure on your local hard drive. Rebuilding the Desktop may solve the problem, or you may have to backup, reformat, or restore the drive.
Why=6 means "overflow trap error" and is pretty general and not much to go on. With Why=6 it's more important to know what else was going on that may have caused a conflict (such as what applications are running and what INITs and cDEVs are on the system). You may be seeing an incompatibility, memory conflict, or just general system corruption. Trashing and reinstalling the System with the Installer usually takes care of this.