This uses to be a permissions problem.

Also use to happen on OOB SharePoint Workflows.

Perhaps the next steps could help ( It 'd be analyzed by your SharePoint administrator )

The issue here is the IIS App pool account and the SharePoint App pool account are not running under same identity. Hence refreshed the app pool identity of the central admin by following steps
 - Check the Application pool identity of the web application from the Internet Information Services   Manager .
    a) Log in to the WFE and then click on "Start" --> "Run", then in the window type inetmgr
    b) Expand the "Application pools" then select the application pool that has the issue right click on it and click on "Identity" tab.
    c) Check the "Application Pool" Identity. Below is the screen capture for reference

        


 - Go to SharePoint central Administration and click on the “Operations” tab. Under the “Security configuration” section click on “Service Accounts”.

 - In the “Credential Management” section select “Web Application Pool” then select “Windows SharePoint Services Web application”.
- Select “Application Pool” from drop down. 
- Select the account as “Configurable” give the service account and respective password. 

- Click Ok! these steps refresh the Application Pool Identity.