I still get it every time.
If I use the other winbox window and press connect it connects.
When I press the reconnect the last thing I see is logging in... then it fades out. Sometimes it returns to reconnect but still doesn't connect or not show up at all
Same problem, same behavior... since v3.14 - viewtopic.php?f=21&t=134940&start=50#p665710
After pressing "Reconnect" button, Winbox logs in, minimizes and hide the window (not visible even on taskbar) and then after exactly 30 seconds automatically logs out, then the same window (connection lost) shows up again. I stopped to use the Reconnect, as it's frustrating and useless at the moment.
EDIT: I found the cause - this only happens when you have "Open In New Window" option enabled. With this option disabled, Reconnect button works as expected.