After upgrading , an issue was found with the field 'Planned End Date' on the Project Task form, showing an incorrect value {"status":"success","date":} for a new project task as well on existing project tasks.

Customized Script include 'AjaxProjectTaskUtil'.

Reverting the script include back to the OOB version.

The Script Include "AjaxProjectTaskUtil" is responsible to calculate the end date and duration for the projects and project tasks, being called by the Client Script "Calculate End Date From Duration".