Zen Cart 1.5.1 (released in 2012) is going to be more difficult to debug, but when you receive a blank-page there "should" be a file created in your store's /logs directory named myDEBUG-adm-*.log that identify what the PHP issue is.
Remember that the -adm- debug-logs contain your store's secret-admin name, so if you post the contents of that file, be sure to xxx-out that name.
Other than some underlying PHP-related issue, I have no explanation for your orders that have an undefined (i.e. 0) value for their status-code ... but that's for a separate forum posting to determine.
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