Mostly this error occurred and related to ABAP editor. The new editor to be precised. You can switch to the old editor and the error will gone. If you would like to continue using the new editor here’s some tip on how to make this error go away.
Go to, C:\Users\<userdir>\AppData\Roaming\SAP\SAP GUI
And rename the “ABAP Editor” folder to something else. Logoff from SAP GUI and relogin. It will automatically create a new “ABAP Editor” folder with fresh XML configs. Previous XML malformed error should be gone by now. Enjoy coding!