I am having the same issue and get the same error message as the O.P.
I was able to add most of my things from ST to Stringify but when I went to create a flow a couple of things are missing. I tried to add these missing things into Stringify but I can't and get that same error message: "Authorizing the selected devices failed; please try again later or contact firstname.lastname@example.org"
I also have a flow with the Ring Video Doorbell not working.
I was going to unkink and relink the Google Assistant but I couldn't get that done.