Same problem. Tried different providers, landline and mobile internet - same behaviour. Tried deleting cache, tried absolutely clean fresh client - same. Tried VPNs - same. When server is full or close to full an getting stuck on "Authenticating" window and getting disconnect after few seconds. During the day when not many players - everything works. Sometimes after many many (really many) attempts I can enter into the queue and login...
I think it started after server restarts on 16 Feb.
Ok, it might not be connected to the server being full - i just experienced same problem and warmane website says that it's only 7K players on Onyxia (my server). But i had only 2 failed attempts and then the 3rd attempt was successful, while in the evenings there can be 20-30 failed attempts before i got put into the queue (maybe even more).
Looks like i got lucky previous time... same situation currently on another account - servers are not full, can't loggin, probably 30 attempts already failed.