I had the same issue when editing the master pages for the 'Companyweb' site on SBS 2011.
It seems to be permissions related and I was able to get rid of the error by logging into sharepoint with the site owner account (as opposed to a 'normal' site editor) and did the following:
- Go to 'Site Actions' at the top right of the page then click on 'Site Settings'
- Under 'Site Collection Administration' select 'SharePoint Designer Settings'
- Tick 'Enable Managing of the Web Site URL Structure'
After doing this the error went away.
I should point out that I'm new to SharePoint and 'just fiddling' rather than dealing with a production site, so I can't comment on whether or not it's actually a good idea to do this or not!