I had this problem few days ago, so I reinstalled the game, updated graphic drivers and the next day it was magically solved.
my brother is having this issue now... When we play together, my game joins with no problems, but he is stuck at the screen with characters windows... Even by forcefully turning off the game and re-login, he can't join us and is stuck at the screen again and then send to the menu where he can't click anything. ...... It sometimes works and he can play a match or two, but then again cant join... It is really annoying for everyone. He is angry because he can't join in and we are sad because we lost one player.
I am desperately searching the internet for any clues that could help now, since we like playing Paladins.