I can reliably reproduce this bug by playing Maelstrom Arena. It never happens in the same spot, but every single run, I get long periods where I can use abilities, but I can't light attack or heavy attack. My framerate is 100 and my ping is stable at 120, so I'm not convinced this is an issue with the performance of my PC or my network. Can the developers try playing vMA a few times and see if they can identify the issue?