Most professional drawing programs like Illustrator allow you to define how a line ends: square, circular,... . PostScript uses the "setlinecap" command to pass this information on to the RIP.

So a PostScript error, offending command "setlinecap" points to a problem with a custom PostScript line or an imported graphic with a custom line. Try opening all epses made in Illustrator, FreeHand,... see whether specific linecaps where used and change those. Resave the files and try printing again.