Steps to reproduce:
1. Run Windelands on Windows (in windowed mode, since you will not see the title in full screen).
2. Start a new game
3. While the game is loading, observe the window title.
On Windows (at least win7) the title will include "not responding" most of the time while the game is loading. Note that the game loads fine and everything works, it's only when the game is loading the title displays "not responding". I am not sure if the intense loading ignoring all user inpit makes the window unresponsive, so that the message is justified, but I fear that users (especially on slower systems) might think Widelands has frozen and won't complete loading. On the other hand I am not sure how much we can do to change this behaviour.
Oh, and I have not seen this on Ubuntu/other operating systems. But since that's a different OS, checking if a program is still responding or not, might be (and probably is) done in a different way.
I encounter the same problem on my Windows Vista laptop. The Widelands screen freezes thereby apparently halting the load progress indicator as well as the OS adding "(not responding)" on the Widelands window bar.
It may not occur if you don't touch the mouse or keyboard during the Widelands load, but the moment I adjust the window position or click away to another window, the Widelands screen freezes and the message is displayed. The load progresses normally, but without looking at my harddrive activity light, there is no other indication Widelands is still working until the map window opens.