sddm crash preventing a second X session

After a clean install of Kubuntu 18.4 I can't open a second X session without crashing X and loosing my current open session.

Step to reproduce

1) Login user01 and get the KDE desktop (K 18.04 put it in VT1)
2) inside KDE, Menu K, Open another session

Results: New login session opens in sddm but second user takes VT1, current session is killed by crash on X.

Expected result: a new login screen in another VTx, with current VT1 still active.

I have the crash report for sddm.

Hardware:
Dell Vostro 3560 (I7)
8GB Ram
500GB SSD in /dev/sdb
1 TB disk in /dev/sda

root@olivier-ntbk:/var/crash# lsb_release -rd
Description: Ubuntu 18.04 LTS
Release: 18.04