"The page has been modified by another author on ..."
when modifying the SharePoint Site Navigation, then it is probably because of a corrupted heading node. If you find the target URL for the heading set to something like "/_layouts/viewlsts.aspx?BaseType=0?BaseType=0?BaseType=0?BaseType=0" then the heading is corrupted.
To fix the problem, you will basically have to remove them, and the header either manually, or using the list settings.
This is a known bug and allot of people have been experiencing it.