Sorry, I forgot there were recent changes around this. Access to xdg-open was just recently opened up as you can see in this conversation.
As an easy workaround, you can try copying the trivial xdg-open script in your snap for the time being, until this change goes live in an upcoming core snap. The script is just a shell line sending a message over DBus.
Please let us know how it goes, and I’m sorry for the trouble there.