You have a broken precompiled header (.gch) file I guess. Try to delete them all.
That was the problem! Thanks!
The strange thing (I don't completely understand how this precompiled headers work) is that I had two copies of the precompiled headers, one at [trunk]/.objs/include, which was up-to-date, and another copy at [trunk]/src/include from february.
I assume that the IncrementalSearch plugin was using the old ones at [trunk]/src/include.
But why the core-plugins were using the right ones? Is the location of the precompiled header settable somewhere? Even if it is so, why is the IncrementalSearch plugin not complaining that it cannot found the files and is now searching in the right location? :-o