We just upgraded from 3.4 to 3.7 We began experiencing 2 frequently with multiple customers after upgrading to 3.7 The errors do not occur with all customers but enough so that our client's customer service dept. is getting buried.
Error 1 Object reference
Error while placing order. Error 1: Object reference not set to an instance of an object. 7/4/2016 5:38:07 PM
Error 2 Duplicate transaction entry
Error while placing order. Error 1: Payment error: Error #E00027: The transaction was unsuccessful.. Error 2: Payment error: Error #11: A duplicate transaction has been submitted.
I recently tried to upgrade from 2.9 to 4.2. I had 10 nop websites that needed upgrading. Getting error upon error, i got some help from fellow nop user named Saurabhdhi. He helped me through a few of the first errors. I eventually just had him to the complete upgrades on all 10 sites. He was amazingly cheap, fast and thorough. He even upgraded my plugins.
Go see saurabhdhir you can PM him from here for help or even have him do it.: