Steps to reproduce:
- Hover over "Wiki" and click on "Administer Wiki"
- Click on "Presentation"
- Click on "Manage Color Themes"
- Click on the default theme
- Click "Edit"
- Hover over the XWiki logo and click on the pallette
- Write another logo name in the logo field
You are able to change the logo, but you'll get an error when you modify that field. I've attached a screenshot.