I'm pretty sure I know what this is - but I don't have a fix for it yet.

It's the function lock_and_write <- it has been incorrectly written.

I have worked around it using file_put_contents but you can't use this if you have more than one admin (as it doesn't lock the file during writing) and anyway it's not a proper fix as it won't work with upgrades.

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

Can you please give me as much info as you have on php / MySQL and server etc. (I need to compare with the servers I have that work, and don't work, with this problem.)