Hey I succeeded to make it work ! everything is OK now.
Steps I took :
-after install, do not forget to put the extra line export KUBECONFIG=/etc/rancher/k3s/k3s.yaml in the .bashrc file of your user (important)
-I ran “sudo kubectl get pod --namespace rasa” to have the list of the pods
-there you can note the name of the pods and the status. In my case the 2 following pods had a status “crachloopback” : rasa-rasa-worker and rasa-rasa-production. (no more on this printscreen as it is done after solving the issue 
)
-type then "sudo kubectl describe pod rasa-rasa-production-xxxxx-yyyyy --namespace rasa "(you can find the exact numbers xxxxx and yyyy of your install in the previous command)
-in the describe, I found the error exit code 132 . After looking it up in the docs, it appears to be an error for non supporting SSE4.2 processor instructions.
-back to my hypervisor, I shutdown the server VM, changed the default emulated processor type to passthrough core i5 8xxx (take any recent processor supporting SSE4.2) and restarted. It then works perfectly for me (flickering is gone, upload model works back, …). NB : if you have just an old CPU (before 2010 more or less) then SSE4.2 will never be supported unfortunately due to hardware limitation.
maybe there are other possible causes but the use of “sudo kubectl describe pod …” will give you the error code you can search for. NB : I have tried with docker-compose also and the problem was exactly the same. So it might also be solved by editing the CPU type. But I did not have a look at error codes.