Another error in the crawl log:
Using Fiddler I figured out that the pages report an “incompatible browser” error if the SharePoint crawler opens that pages. It seems that the gantt view only renders content for an up-to-date Internet Explorer – identified by the user agent.
The solution is to change the user agent used by the SharePoint crawler component.
Microsoft’s “procmon” showed me this registry key:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office Server\14.0\Search\Global\Gathering Manager\UserAgent
I set this to the IE9 user agent string found here: http://blogs.msdn.com/b/ie/archive/2010/03/23/introducing-ie9-s-user-agent-string.aspx
Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Trident/5.0)
After the next crawl the errors related to this problem where gone.