I was just playing Arena against a player named Bobo.
First turn I played Voidwalker.
2nd turn I played Faerie Dragon, but couldn't attack with void walker. No highlight effect appeared until briefly when I kept clicking on it, but I still couldn't do anything with it. At one point it selected it but the attack UI only appeared for a second.
I couldn't even hit the end turn button.
Same thing next 2 turns. I couldn't attack with any units, but could place a single card on the board. After that (in that turn) everything stopped highlighting and working. No hero power, attacking, playing other cards or ending turn. I could use speech commands and also see the opponent highlighting stuff, and speaking.
So yea, pretty frustrating. I wasn't able to concede either, the button did nothing. I had to actually quit the client. When I reopened it Bobo wasn't listed as my last opponent but it did manage to record it as a loss...