So, seeing this error?
The user interface of the master page cannot be determined. Do you want to continue?
In SharePoint 2010 the master pages can be assigned to User Interface (UI) Version 3, 4 or even both. What this error message means is that your master page has not been assigned a UI Version. This is a super quick thing you can fix.
From your site:
- Go to Site Actions → Site Settings → Master pages and page layouts (under the Galleries column).
- Locate your master page in the library and hover over the file name; the sub menu arrow will appear.
- Select the arrow then select Edit Properties. If you are prompted, check out the file.
- Check that the master page file has been assigned to UI Version 4 (or 3, depending on your needs).
UI Version 3 refers to SharePoint 2007, UI Version 4 refers to SharePoint 2010. To help you remember, just think about how many versions of SharePoint have been released.
- 2001 = version 1
- 2003 = version 2
- 2007 = version 3
- 2010 = version 4
You would set a master page to UI Version 3 for SharePoint sites that have been upgraded from 2007 but have not been visually upgraded yet. If you are working with a spanking new 2010 site, there is no need to use UI Version 3.
Thanks to SharePoint Experience alum Patrick for the blog post inspiration!