Xerxes
His old name was 16 characters long which in game was cut to 15 characters long.
He changed his name to only six characters (even has a hyphen where the original was just alphabetical, no numbers or special characters) and was able to pass the verification check in all servers. This leads me to believe that the hash for the challenge works so long as the game's characters for the username directly match to what Plutonium's username is for the user (if the game limits certain character sets, cryllic, unicode or otherwise, the hash may continue to fail for people with shorter names). It might not be a bad idea for people having this issue to change their Plutonium names to something short and alphanumeric until the hash that runs the challenge is fixed.
Should be noted that he was on a non-steam version and had to manually designate the T6 apps in his firewall on Windows. Even before the name change, he was able to connect to some servers after allowing the app through the firewall, but not all servers (specifically, not mine which only runs IW4MAdmin and bot-warfare). Not sure if there's a plugin somewhere that allows for longer usernames, but I noticed in my server readout that he was failing the challenge check and asked him to change his name (he had the longest name of anyone I've ever seen on Plutonium, and I noticed his launcher had the full name whereas it was truncated in-game).
Hope this helps.