Hi Yves,
If you changed the hostname of your SAP System, you might need to do the following (you can perform this on a running SAP System):
Go to transaction code SM51. Choose Goto > Host Name Buffer > Reset. At here, you can select either Application Server, Message Server, ICM or Entire System.
Go to transaction code SMGW. Goto > Expert Functions > Host Name Buffer. Click on dustbin icon or go to List > Invalidate Buffer.
This way you are sure that the new SAP hostname is "known" and the connection in/out are also using the new hostname.
I hope this will help you.
Regards,
Andre