Found the problem... (admin/doeditconfig.php)

I believe there may be an issue with the ubbthreads function lock_and_write (I haven't found it yet but will keep looking).

This was the offending code block. (n.b. I have changed he name of config.inc.php for testing.)

Code
$check = lock_and_write("{$config['FULL_PATH']}/includes/config2.inc.php",$newconf);
if ($check == "no_write") {
	$admin->error($ubbt_lang['NO_WRITE_CONFIG']);
 } // end if

Whilst looking into this, I have replaced the code above with my code below.
I don't advocate doing this as there is no file lock.
I am simply reporting to keep this thread updated for anyone else with problems.

Code
$check = file_put_contents("{$config['FULL_PATH']}/includes/config2.inc.php",$newconf);
if (empty($check)) {
$admin->error($ubbt_lang['NO_WRITE_CONFIG']);
}


This holds true also for custom islands etc.