any chance anyone knows why this is occuring?

NetSuite support might be able to look it up

From experience, when you open a ticket with NetSuite, send them the URL from the page that displays the unexplained error message.

You can try a different browser, sometimes edge/firefox gives a better error than that to help pinpoint what is causing it

It's possible there's something wrong with the template that's used to print the SO. Besides reaching out to NS support, I'd recommending taking a look at the PDF template to see if any new changes have caused this issue. Sometimes it's best to see if a vanilla template for SO gives you the same error and then via trial and error edit the existing template until the error goes away.