EDIT: Look at the end of the message for solution.
I have the same problem. The background image of the header module doesn't show up. Things I've tried that don't work:
- Changed the image to the default and to other images in other paths
- Created new header module and replicated all the module info from another clean installation
- Removed all the custom HTML code from the module (empty module)
- Emptied all override.css entries
- Unpublish / Publish
- In Options tab, toggled the Prepare Content on and off
- In Advanced tab, toggled caching to off and use global
- Toggled language (All, Lang1, Lang2)
- Toggled the showing of the title show/hide
It seems that whatever I do, the background image of the header module doesn't work

I don't know what else to do....
Solution: I went to Extensions > Templates > gk_events > Features tab > Suffixes for pages > added rule:ItemID/Option 640, Suffix frontpage top1-colorbg > Add rule > Save
It seems that somehow this entry was accidentaly deleted.