My guess is that while it tries attaching the first client, the second client got lauched too and SIN failed to attach the second client. I see only one tab in SIN and then the Attach Error dialog box. Note that all clients are lauched successfully.
However, the Attach Error Dialog box cannot be accessed for some reasons. This make SIN Window became inaccessible.
To workaround, I right click on the multiple window icon of SIN, which is now in the taskbar, and choose "Close all Windows" option. The Attach Error dialog goes away and SIN now can attach all the clients and show tabs for all of them.