Just to add...
I spent a few minutes looking at this, thinking that maybe the password had been "reset" to a default.
For whatever reason, there's something wrong with the password hashing now.
With my original (9-character) password, the SHA1 encoding stored in settings.json is not recognised.
Changing my password to a 14 character password worked.
Doesn't matter if it's "<originalpassword>12345" or "aaaaaaaaaaaaaa", the SHA1 hashing works and I can login... 13 character or 15 character password, and nope, unauthorised.
Looking at the instructions, you should be able to stop transmission daemon, enter a plaintext password in settings.json, restart the daemon and then login... but some of the hashes are just plain wrong.
As transmission hasn't changed, then it must be something in the new packaging that's getting warped somewhere.
Anyway, I hope changing your password to 14 characters helps