I also had this problem. It appears when using ProtonVPN and OpenVPN as the connection, you MUST set the VPN in Deluge settings to Custom, not "proton", this breaks the WebUI for SOME reason. I wish I was of more help, but ProtonVPN is just insanely slow for me as well. I cannot for the life of me figure out how to make sure its using an open port and p2p.