When installing Tiki 4.0 (or even 5.x or 6.x) with PHP 5.1.0 (or higher), you may encounter a System error: could not find driver message. This indicates that PDO driver for your database has not been enabled on your server.

To enable the driver:

• For Unix:
  Refer to http://us.php.net/manual/en/book.pdo.php for information on installing PDO support.
  As an alternative, add the following statement to the end of your ../db/local.php file, before the ?>:
  $api_tiki='adodb';

• For Windows:
  Add the following statement to your php.ini file:
  extension=php_pdo_mysql.dll

• Tiki 4.0 doesn't like spaces in URL so some.site.tld/My Project/ session/cookie doesn't match and users cannot login anymore (this worked fine in 3.x)
• After upgrading to 4.0 from 3.x, new user registrations may not be properly validated. This was fixed with SVN commit #23514 (or pick the daily 4.x built on de.tiki.org) . If you still experience problem (Disabled account) you can check the table users_users and run the sql query

update users_users set waiting=NULL where waiting is not NULL and valid is NULL;
If you want to do only on my account foobar
update users_users set waiting=NULL where waiting is not NULL and valid is NULL and login='foobar'

See Tiki4.