It's an issue with the file caching system. Basically, SMF determined that the file existed, but it didn't exist by the time that SMF tried to include it. As far as I can tell, it can only happen if two users try to access the file at almost the exact same time, right around the time that the file expires.
The only way I can think of to get around this issue (other than asking your host to install a PHP-based cache such as APC) is to make a slight change to the code:
require($cachedir . '/data_' . $key . '.php');
@include($cachedir . '/data_' . $key . '.php');
This change does two things:
1. Using include() instead of require() will prevent the script from dieing if the file can't be included.
2. The @ will suppress the warning generated by include() if the file doesn't exist.
That should solve your problem.