9.2.1 Rest API Change a batch of single entities not working as expected

Thanks for the reply, Markus. I should have specified. I am doing a post request.

Invoke-RestMethod -Uri $reqUri -Body $jsonOutput -WebSession $wsession -Method POST -ContentType application/json -Headers @{Accept="application/json"}

Good catch. I think the documentation is wrong, and you need to use a PUT instead of POST at least according to the API documentation page in the Application Server itself?

Can you test if using a PUT solves your issue? I will contact the documentation team then. 

Using the same JSON, and this:
Invoke-RestMethod -Uri $reqUri -Body $jsonOutput -WebSession $wsession -Method PUT -ContentType application/json -Headers @{Accept="application/json"}

Returns this oddly formatted message:
"errorCode":"NotFound","message":"Entity with key ACCProduct982f3ec0-0b36-42c4-9d3c-fdea773b5edd was not found."

I just rechecked to confirm that that UID corresponds to a valid ACCProduct entry. I am not sure why it is combining the UID and the table name in the message.

Using the same JSON, and this:
Invoke-RestMethod -Uri $reqUri -Body $jsonOutput -WebSession $wsession -Method PUT -ContentType application/json -Headers @{Accept="application/json"}

Returns this oddly formatted message:
"errorCode":"NotFound","message":"Entity with key ACCProduct982f3ec0-0b36-42c4-9d3c-fdea773b5edd was not found."

I just rechecked to confirm that that UID corresponds to a valid ACCProduct entry. I am not sure why it is combining the UID and the table name in the message.

I tried to reproduce this with a 9.2.1 build. I can confirm that you must use the PUT method. But my error message looks like this (pure JSON return value)

{
  "responseStatus": {
    "errorCode": "NotFound",
    "message": "Entity with key <Key><T>ACCProduct</T><P>982f3ec0-0b36-42c4-9d3c-fdea773b5edd</P></Key> was not found.",
    "errors": []
  }
}