I am presuming that the date is set correctly on your server?
These should be generated automagically every month, although I'm not 100% sure which process does this.
Is it on all of your streams on the server or just one? If you have the folowing setting:
define('QUOTA_SPOOL_ONLY',false); and the person has used virtually all of their disk space allowance then there simply won't be space to store the log files. I tend to change this setting to true, but that is totally up to the host.
These can also be found on the server in the following location: