LogMeIn failing to display log-in screen for certain computers

I have a colleague on a mac who is unable to connect to two of the machines in our account. I'm able to connect to those machines, and he is able to connect to others; just not the two he needs to (and has been using previously). The problematic machines are a mac and a PC, and he is able to connect to other macs and PCs. He's tried using my log-in details but that doesn't work either.

The most odd thing is that when he clicks on the control icon, nothing visibly happens but in the Window toolbar, the name of the computer he's trying to connect to appears, as if the client is opening the window to connect but not displaying it. He is on the most up-to-date client and we've done a fresh install.