WordPress MU

Hi-all
when I change "Home" URL in edit blogs section and click to view edited blog.I get a 404 NOT found error. the blog shows correctly(with same name,same sidebar) but with no post! (even feed,categories or archives has not any post)
Why?! Is it a bug?
I could change it(to any thing) before. when I did it all permalinks URL change to new address(ex: for about page from user.domain.tld/about/ to sub.domain2.tld/about/)
Note: that's not important for me new address really works or not. I just want to change urls in front page of blog.(is there another way)

Thanks

maybe this is the same problem
http://mu.wordpress.org/forums/topic.php?id=6433&page&replies=3

I believe the URL is listed a total of 4 times within the edit blog page. Be sure to edit them all.

Thanks for the reply. I changed all with no success.(I use subdirectory)

another thing. I changed permalink structure for a blog and get the same error.
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I deleted permalink structure (to use domain.tld/?p=1) and now all things(include Home url) works fine.

I'm mixed up!