I'm having this exact same problem. DMC is not open in another window. Problem persists even when I shut down the service in task manager and then restart it.

I tried disconnecting my phone and PC in the app, and reconnecting, and that seems to have solved the issue, at least for now.

I have exactly the same problem "Dell Mobile Connect is open in another window". Nothing happens when I try to "use this window". I tried restarting my computer and reactivated bluetooth, but neither helped. I then uninstalled and reinstalled the app on my phone, which temporarily worked. But as soon as I closed my PC and came back later, the same error message appeared. 
Does anyone have a suggestion how to resolve this? Thanks.

I am having the same problem.  Went back to using the Microsoft version of this product. 

I am having the same problem and the fixes mentioned do not work. I spent over an hour on the phone with a agent from Dell and it was working but when I restarted my computer the same message appears.

My dell XPS15 9570 started experiencing the exact same symptoms yesterday.

This worked for me. Disconnect phone in both apps and rescanned the QR code

i opened the app on the phone.  from menu, dissconected it from computer.  then reconnected using the scan

Same thing happened to me and the disconnect and reconnect process worked.

I've noticed this app continually dropping connection, usually it reconnects, very often it takes a long time to reconnect.  This time it would not reconnect on its own.

I have disconnected and reconnected both on my phone and computer. It fixed it for 2 days and it is right back to Dell Connect is open in another window.