Is the same without back attached? I know you can not fire without back and film in it in single exposure mode - but just to rule out back problem. Did you test with body only, nothing attached (no back, lens, finder even focusing screen? I tested it just now and with nothing attached to the body
shutter lock OFF (9 o'clock position while looking from the front of the camera) , multiple exposure in OFF vertical position - i can not fire shutter (which is normal bc no hack with film in it was attached) but can fully depress it

Shutter lock OFF, multiple exposure in ON horizontal position - i can fire shutter