Error of "Object Reference Not Set to Instance" while doing Special Order Payment at POS

Error of "Object Reference Not Set to Instance" while doing Special Order Payment at POS

Environment:

iVend 6.6 with S4 HANA

Problem statement: 

A Special order is created  from one  store  setting  the delivery warehouse as the other store where the order needs to be delivered.The special order is created successfully, however while doing the Special order payment from POS, it is throwing an error "Object Reference Not Set to Instance". The Stores are also using Transaction Server Search

Symptoms:

 NA

Resolution/Work Around:

Need to check various data and information.
1. Check the TrxTransactionOrder details on HO db and cross check all the data in it. 
2. Check the data in the Integration_Transaction SP and validate it.
3. If all the data seems to be correct in the db then check the API connection at both HO server and store server.
Browse the API through IIS on HO server and check if it is running fine.
Then copy the URL of API and then browse it at the Store server and it should be running fine there also.
4. Go to the Enterprise screen at HO MC and check the API settings are correct or not.
5. Shutdown POS- > rename the POS Log file-> Restart the POS and check the LOG.
In this case, the error was removed by Restarting the POS and it seemed that after restarting the POS the data was fetched on time and thus the Special Order payment could be done.

Recommendations /Suggestion:

NA