In your /cache/ folder will be several files named mydebug-xxxxx.log, where xxxxx is a random number, in date order.
In those files are the PHP errors that are being triggered when the blank screen appears.
The blank screen happens because you don't want the "actual" error message displayed to your customers, and certianly not to hackers. Hence the fallback to logging instead, since only you have access to those logs.
Find out the errors that are happening, and then you can solve your blank-page problem.