Game client - new arena feature

[ILLINO3]

For some reason, I cant que arena anymore. It says, I use modified game client. I tried to delete my addons, update, WTF folders; even downloaded a new game client from wow-circle website. Doesnt seem to work for me. Any suggestions how to fix this issue?

Edit: I tried my second account, and what I found out is that, on some of my characters I have no difficulty to join arena game. BUT then I tried different character ON SAME ACCOUNT and it didnt work out.