Thank you for using the Support Forum. It seems like you have narrowed it down to an issue with the Apple Mac Mini with the M1 processor. The printer is printing for everything except the envelopes from the Mac mini so the printer is working. You state the Easy Envelope program is the same on the two Macs. I would suggest you check with Apple support to see if they know why the envelope printing would not be working on the MAc Mini. Please consider contacting your support centre for further assistance.
The new Apple Mac Mini with the M1 processor has some inconsistances with printing. I have a new Mac Mini with an M1 processor and I have a 16" Apple MacBook Pro with an Intel i9 processor. Both units are running OSX Big Sur v11.1. I can print envelopes using the 3260's manual feed slot with my Intel computer but I cannot print envelopes using the M1 Mac Mini. When printing envelopes from the Mac Mini the printer produces a red-light fault.
I have downloaded the latest AirPrint 3260 driver to both computers. The envelope applications (EasyEnvelopes v 1.0.2) are the same on both the Mac Mini and the MacBook Pro.
The 3260 will print normal documents from the 500 page paper tray properly.
How may I fix this problem?