A new idsite can't be archived, can anyone help?

I add a new idsite 27 to my piwik server. But it can't be archived by archive.php. I think it's not the memery limit problem, I have set it to 4096M and restart my apache server. Errors like below:

[2013-08-02 06:26:35] [483646ff] Archived website id = 26, period = month, 889 visits, Time elapsed: 0.511s
[2013-08-02 06:26:36] [483646ff] Archived website id = 26, period = year, 1949 visits, Time elapsed: 0.941s
[2013-08-02 06:26:36] [483646ff] Archived website id = 26, today = 12 visits, 4 API requests, Time elapsed: 2.406s [24/25 done]
[2013-08-02 06:26:36] [483646ff] ERROR: Got invalid response from API request: http://w1.ce.shanlink.net/piwik/index.php?module=API&method=VisitsSummary.getVisits&idSite=27&period=day&date=last52&format=php&token_auth=45b0b7014c963ff03059f3f67107b46f&trigger=archivephp. The response was empty. This usually means a server error. This solution to this error is generally to increase the value of 'memory_limit' in your php.ini file. Please check your Web server Error Log file for more details.
[2013-08-02 06:26:36] [483646ff] WARNING: Empty or invalid response '' for website id 27, Time elapsed: 0.514s, skipping

 What can  I do about it?  The idsite 27's visitor log has been recorded in piwik_log_visit, but can't be archived.

What error can you see in your server error log ?

Thanks matt, I haved solved the problem. The error occured because I modified the plugins/Actions/ArchivingHelper.php file and created a bug in this file. After I fixed the bug, the archiving error disappeared.

What error can you see in your server error log ?[/quote]