When I do the port forward on my own using 

ssh -L

 also seems to fail for jupyterlab and vscode, too, which i find odd

The only external port exposed is the SSH one 10022, then the client forwards it locally (so you, the user, can always have the same connect i.e. "ssh root@localhost -p 8022")
If you need to expose an additional port , when the clearml-session is running, open another terminal and do:
sh root@localhost -p 8022 -L 10123:localhost:6666This should port-forward the container's internal 6666 port to your machine's 10123