A pure white page means a php code error. If you have access to your server's error logs through Plesk or CPanel then the file at fault will be listed in there. If you don't then contact your host and see if they can give you the file.
Because it goes away and doesn't happen every newday, tells me that it's probably a "newday-runonce" problem and the error is probably something simple like a misspelt function name.
You could start searching for modules that hook to "newday-runonce" and examining the "newday-runonce" hook code, but if it is a misspelling then it'll be very hard to find.
