Steps to replicate

• Login as Admin
• Go to Site administration > Appearance > Themes > Boost
• Add an image in Background image and in Login page background image.
• Click on Save Changes
• Reload the page and wait until the following icon appears, which is the icon that appears before the image preview.
• Log out in that exact time
• Login as Admin
• Check that an image appears instead of the home page. 

This issue may happen when the connection is slow and the user logs out when the file manager is still loading.