The reason of the problem was so silly (and so obviously solvable) that I'm almost embarassed by having had that issue.
Quoting from the KDE Community Wiki
I keep being told "Authentication Failed"
Make sure you have telepathy-auth-handler installed.
Do you have Empathy installed? If so remove it.
Also check all the steps in the following section.
telepathy-auth-handler
was present in my system, and Empathy is not installed, so I went on:
Google Talk disconnects with Network Error shortly after connecting
You need to check if your avatar (user picture) is small enough. Maximum file size allowed is 8192 bytes, maximum allowed dimensions for avatars are 96x96.
After having changed the avatar picture to a proper one, the account works properly.