I installed Riot from the repositories and tried to login using Single Sign On. The way Riot handles this is opening a link to the browser to let you login there, and then you should get a riot:// link that sends you back to the desktop application.
However, the application from the repositories does not register itself as an handler for that "protocol", the link is ignored, and the login fails (in a quite cryptic way too... You simply get a black page in the browser and no error whatsoever, while Riot is just there waiting).
For me the fix was to add
to the desktop file (copied from the flatpak version) and then updating the desktop database, but maybe I'm missing something.
This is just a temporary fix, however, as it will be overwritten at the next update.