Since yesterday night (July 1), it seems that LINE has been blocked by the Great Firewall of China. Without a VPN, it's possible to see that messages are coming in (alerts are shown), but messages can't be read nor sent. A VPN allows access as usual.
Hoping that this will be only temporary but I'm not optimistic...

Some reference articles:
http://shanghaiist.com/2014/07/02/li...p_banned_i.php
http://www.techinasia.com/japanese-c...ainland-china/