Yup, that one's known, although I forgot where it was posted.
I worked around it by changing the size of the 'back to main menu' to 1/3th.
In MainMenuGUI.ini;
Code:
[ButtonBackToMain]
ControlType = BUTTON
Parent = root
X = 260
Y = 280
Width = 40 // 120
Height = 20
Visible = True
Enabled = True
Name = ButtonBackToMain
Anchor = Left, Top
Text = Back //Back to Main Menu
for those who'd like to play in 800x600.