I have just tried both things you suggested and unfortunatelly it didn’t help. I think this issue is due to WordPress itself.
The only thing that comes to mind is that I changed Permalinks and .htaccess accordingly. The new permalinks settings work well, even for older posts to which visitors are referred from Google. I did it with the help of the plugin called “Dean’s Permalinks Migration”.
However I don’t think this should affect the search function.
