In the console, the user can find more details about the error. The error could be because of the Client scripts that are running on the form. Go to that script and troubleshoot from there. In order to open the browser developer tool console,
1)Right-click anywhere on the form.
2)Click on Inspect
3)Click on the Console tab
Here find the Client script that is causing the issue. You will either need to update this script to work on both portal and backend views, or you will need to write a different script to run only on the portal, and leave the current script to run only on Desktop.
There are a few methods that are not supported in mobile/service portal but works fine in native UI. Avoid such methods while creating Client scripts for the portal. Refer the following product docs that explain more about unsupported methods.