The issue is with the update server behavior. You have that field set to "Ignore" if blank, which doesn't allow you to actually update it to a blank value.
On line 364 you have:
$UpdateQuery->bindColumn("document3", "s", "".$WA_DFP_UploadStatus["WA_UploadResult4"]["serverFileName"] ."", "WA_IGNORE");
I think it would work if you had:
$UpdateQuery->bindColumn("document3", "s", "".$WA_DFP_UploadStatus["WA_UploadResult4"]["serverFileName"] ."", "WA_BLANK");