As a workaround…
Assuming you are in local Duplicati, try changing everything after the port number (probably 8200) to theme.html to see if that does any better at getting the old UI.
Your referenced ticket tells me that this is 2.2.0.2_beta_2025-11-26 which had an undocumented UI change that looks like it added this to Settings. Is that the spot?
Problems with your authenticator logic is why I asked to confirm you’re in local UI.