Facing 'The system has encountered an error. Please try again Later' HTTP Code: 403

Hello,

I am facing the below error while completing one task by clicking on a button.

Also, I am getting the below error for the same task while I am trying to complete the same task using the Process Model's User Input Task's 'View Form' option:

The process is getting stuck on this user input task without throwing any error. This error has been occurred after Appian 21.1 release. Is it something related to the new release?

Please help me in resolving the issue. I can't post the code here due to some security restrictions.

Thank you in advance.