BPM Task API Approve action is throwing an error

Summary:

Need to call bpm/api/4.0/tasks API to approve the resignation approval workflow. First step we are reassigning the workflow the integration user and then trying to approve the task using the same integration user. We are getting the below error when trying to approve the task.

"detail": "Identity service cannot get users when identities size crosses limit   Fix: verify server log to find the problem cause",    "o:errorCode": "76012",    "o:errorPath": null,    "status": "400", "title": "Identity Service cannot get users

Content (please ensure you mask any confidential information):

/bpm/api/4.0/tasks/{taskId}

{    "identities": [        {            "id": "integration.user",            "type": "user"        }    ],    "action": {        "id": "APPROVE",        "type": "SYSTEM"    }}