It's so mainly because of the phpThumb API. It can't auto-detect default e107 cache folder (or maybe I don't want to give a try because of performance reasons) as it don't uses the e107 API. You could try to edit your phpThumb.config.php Additional - Gallery is checking for suspicious files hourly while you are in the plugin administration area (file types not included in filetypes.php) just like e107 core does on main administration screen. Try to copy some .php file to sgallery/cache to see the result.
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