It seems that if I have a Putty session active, alt-tab away from that window and then alt-tab back, the window is not displayed. You have to click on the task bar icon twice to recover it.
Unsure if this is related but if a window is visible in the background behind another window, clicking in the putty session space does not switch back to mRemoteNG, clicking on another part of the window outside of the putty session does.
I believe we fixed this for v1.74
Actually, the issue still persists in the version 1.74. It's reproduced at least in Windows 7 x64. Please consider to reopen the ticket.