I’ve narrowed it down to the table prefix now (not multiple installations) but still not sure what the problem is.
If I install a single blog on a new DB with the default “wp_” as the table prefix, it works fine.
If I install a single blog on a new DB but change the prefix to “wp1_”, then I can’t log in to the admin area, even though wp-config.php has “$table_prefix = ‘wp1_’;” in it!