Ugh, I feel your frustration. Printers always seem to act up the moment you need them to do something slightly different. You did everything by the book—no reason it should jam like that.
I had a similar issue with envelopes on my previous printer, and weirdly the only thing that helped was loading just one envelope at a time and pushing it slightly forward beyond where the guide marks say (not too far, though). It's like the printer needed a little "head start." Might be worth a shot if you haven't tried that already.
Also, on a totally different note—if you ever need a break from battling with office tech, check out Dairyquenmenu.com It's my little corner of the internet where I keep up with DQ deals and updates. Way less frustrating than printers, I promise
