Never had this issue but searching for it at ut99.org came up with a couple possible solutions:
https://ut99.org/ut99.org/viewtopic.php?t=12198
https://ut99.org/viewtopic.php?t=12551
One thread suggests that it might be caused by trying to play with a joystick connected to the computer and/or removing the joystick keybinds in the ini even though these keybinds really shouldn't cause that issue.
In the other thread the person solved their issue by disabling Windows DEP (Data execution prevention) either for just UT or entirely, I'm not sure which.
Are you running the latest patch or at least v469d? And I'm just assuming you run Windows.