Access to Charts in Siebel not possible anymore - License expired or invalid

Dear Oracle Team,

our clients have NetCharts Server 4.6 installed on a production machine.

When we try to access to the NetCharts Server start page (http//localhost:8001/index.jsp) we see the following lines (a screenshot is also attached):

Your license for NetCharts Server has expired or is invalid.
Enter a valid NetCharts Server Serial Number:

Our clients could see their Charts on the Siebel application before a change was made: IP was changed. After this change they couldn't see them (the Charts) anymore, because the Chart Server parameter in Siebel was pointing to the prior IP and port. Now this is updated to the current ones, but they can't still access to the Charts because of a supposed expired license.