Most of the time this is caused by the PHP memory limit being reached. A variation for example with 'size', 'colour', 'shape' and 'style' as the categories can cause alot of combinations which can quickly chew up the allocated PHP memory available.
But in some cases this can also be caused by the Suhosin module that is sometimes compiled with PHP. You will need to consult your hosting provider to see if it is. If it is compiled in then you will also need to ask if there is a PHP setting called suhosin.upload.max_uploads.
If this setting is set then basically this will limit the amount of upload file boxes that can be submitted (the button you click to upload a file). If the maximum amount is reached then the process will stop and a blank screen will be displayed with no product changes being saved.
If you do have this setting then you will need to ask your hosting provider to change the value of the setting to something that will never be reached, like 1000 (there will never be a case where 1000 upload files boxes will be displayed on one page).

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