Hi Christophe,
What OS/browser/plugin are you running ?
Could you send a fresh softphone.log to
jabbersdk-support@cisco.com ?
I am not very familiar with extension mobility, a few things to check:
[1] Is CUCM 10.248.127.140 running the TFTP service ?
[2] In CUCM admin, open the user page. Is the "allow control of device from CTI" checkbox ticked (Extension Mobility section) ?
[3] If done manually, verify the configuration of device "DP_dts8" in CUCM ? Any other similar device to compare with ?
[4] Any firewall or network configuration which would block TFTP (port 69) and/or HTTP (port 6970) between the client and CUCM ?
[5] Can you successfully login to the same CUCM, same user, in softphone mode and/or deskphone mode controlling a different device (ex: hard phone) ?
Cheers, Thomas