1)If you enter a field wrong, the screen refreshes and the error is not visible, because it's displayed in a black background with also black fonts.

("The passwords you entered did not match.")

This has to do with template fonts. You can fix it easily by going to the phpbb template folder and editing the file that has to do with registration. More help can be provided through phpbb forums.

2)For Yahoo e-mails, the registration e-mail with username and password goes to the spam folder. I think everyone can help with that by clicking at the "No spam" indicator.