are you connecting to a protonvpn endpoint that supports port forwarding?, note the section in the webpage i linked to describing this exact error.
that would be a nasty cludge that i wouldnt be happy to entertain, sorry, you would be better off simply using deluge (no vpn) and routing it through gluetun in the way its supposed to be used.
i honestly dont know how gluetun works in this regards, but the word 'killswitch' suggests to me that gluetun is reactive (kills connection when vpn goes down) as opposed to proactive (my images - protection is always on irrespective of state of vpn).