Basic users can't create a blog if another blog with the same urlslug already exists.

Cause:
URL slug automatically gets changed to e.g. "blog-1" but the error message doesn't let user to create a blog anyway.

Solution:
Change message type to "note" instead of "error"
/inc/items/model/_item.funcs.php:311

$Messages->add( sprintf(T_('Warning: the URL slug has been changed to «%s».'), $urltitle ), 'note' );