So your code is running on POS and it's calling your D365FO?
Then something is different in this service or service call compared to the others. You are the only one who knows what is different, unless you share more information with us.
If I had such issue, I would share the code of the service itself, as well as the code that is calling it. And for comparison, I would share the same information about another service that is working. Could you do that, too?
Which line in the code gives you this error?