I'm trying to print #10 envelopes from the main tray of the 7740, getting paper jam errors on the computer and a "paper problem" indicator on the display. The envelopes are fine, the tray isn't overloaded, and when I switch the main tray to A4 paper it works fine. Paper size is set correctly. Resetting the printer, either through the power button or the plug, doesn't change anything.
The machine is MacOS 11.4 (current Big Sur), printer firmware is 2113B (current)