Sub-domains

I have two installs of wordpress on one domain, with the plugin active on the main http://www.xxxx.com domain. For some reason however the sub domain wordpress install (without the plugin installed) will not login xxx.xxxx.com/wp-login

I have to deactivate the main one to access the sub-domain install, is anyone else getting this?

http://ww.wp.xz.cn/extend/plugins/better-wp-security/