Facebook registration leads to nowhere

When I want to register using FB registration form I then get redirected to the same registration FB page like nothing would have happened.
I'm suspecting htaccess. So I would like to know what htaccess inputs must be there for this plugin to work and what can't be used.
If a user is logged in FB and want to login to my page, if he/she provides his/her blog account details and press login with FB then FB account will be synced with this account, so because of this I assume everything is ok with the plugin and problem has to be somewhere in the htaccess.