As a temporary workaround you could just disable the php error messages so your users do not see them.
Just place the following line in the .htaccess file in your forum root:
Code:
php_flag display_errors off
As for the error you are getting, I suspect you are using mysqli instead of mysql. There is nothing wrong with that, actually it is the preferred way with php5, but the memberlist code works only with the old mysql. I haven't tested this though, just looked at the code.