We have a large Nintex responsive form on SharePoint 2016 on-prem. It was working fine for few months. However, nowadays when the user fills all the fields and submits the form,

• It takes very long time and finally it just ends up showing a blank page. (the record does not get saved)
• Sometimes, it loads for a long time and asks for credentials.
• Sometimes, it takes long time and gives an error: "Server connection closed. Description: The server requested closed the connection before the transaction was completed."

Attached all the screenshots supporting the problem. Please help solving this issue.