Error in Customizing - ESS UI 5 Leave Request

Dear Experts,

We are getting the below error while clicking on the ESS UI5 Leave Request. We didn't do any changes in the Leave Request application for a long time.

It was working fine till now. And now it is not opening the error message.

Please give your valuable suggestions.

Error in customizing. Contact system administrator

Awaiting your kind response.

Thanks,

Nalla B.