Thinking more...

升級 3.5 以後，要 Update network，卻發現下面這個錯誤。

Warning! Problem upgrading http://domain.tld/blogs/blogname/. Your server may not be able to connect to blogs running on it. Error message: SSL certificate problem, verify that the CA cert is OK. Details: error:14090086:SSL routines:SSL3_GET_SERVER_CERTIFICATE:certificate verify failed

是沒錯，因為是自家用的網站，CA cert 是自己簽發的，會有驗證有問題的情況是正常。但為什麼之前都沒有這問題，怪。總之，後來查了好一會，知道應該是跟 sslverify 有關係，就找到 wp-admin/network/upgrade.php 裡呼叫 wp_remote_get 的地方，為第二個參數加上 sslverify => false 就可以了。

$response = wp_remote_get( $upgrade_url, array( 'timeout' => 120, 'httpversion' => '1.1', 'sslverify' => false ) )

參考資料：

• WordPress › Support » network upgrade failing on ssl only site with self-singed cert
• WordPress › Support » Error upgrading Mu site: SSL cert problem