I did not install it through Flatpak and ran into the same error. I basically downloaded the .deb form the website. Wappler was not allowing me to authorize, I was stuck in a continous loop of getting back to the auth client, then to the website and then back again.
The only thing that helped was what @Cheese stated here:
You have to maneuver to
/usr/share/wappler-beta
And then you have to type:
exec Wappler Beta
If you have installed a non beta version, you would have to do the same, just without the beta tag I guess.
I don't know what caused this, but there must be something else causing this issue, besides Flatpak being sandboxed. It happened to me outside of Flatpak, as I have not even touched Flatpak at all.
@George