This is the same issue which happened last month. And we found the problem is because of user use " Item Move " function where it duplicated the record in the sales transaction. Once got 1 transaction got stucked, the other transaction will not process to the XnPOS portal.
We have come up a fix which can solve this Item Move issue and you may require to upgrade the Live site.
I will personally send the new version to you for validate. We need to validate this carefully before we deploy this to the Live site.