Comment 10 for bug 1662145

This is not the original issue, but yesterday I switched the unity8 snap to use a different, more standard path (/run/user/xxx/mir_socket) for the socket.

So if testing against the u8 snap, please re-create your app snaps to pull down the latest version of the desktop-launch script which will point your app at the new, correct socket location. Again, this isn't the same issue as OP, but I didn't want this new issue to confuse your testing of this bug.