All of these are soft links to the corresponding file in sites-available.
The problem happens when the phpmyadmin.conf links to itself or somewhere not exist.
Then I found 000-default.conf is loaded correctly because the mod rewrite(written in 000-default.conf, rewrite http requests to https) works. But https requests(https://domain) from chrome received ERR_SSL_PROTOCOL_ERROR. Curl got SSL23_GET_SERVER_HELLO:unknown protocol(on MSYS) and SSL3_GET_RECORD:wrong version number(on ubuntu). It works when I changes the request URL to 'http://domain:443'. That means default-ssl.conf didn't work.
All works correctly after I fixed the soft link.
It's my mistake, I see. But to my surprise, apache2 didn't tell me any warning or error message. And no error logs(/var/log/apache2/error.log and /var/log/apache2/access.log). In my opinion, apache2 should report the config files cannot be opened. I don't sure whether I should add an issue.