Hey, thanks for the report. This is a known issue. The sandbox preflight check crashes the Extension Host instead of degrading gracefully when user namespaces are disabled.
If you have sudo on the HPC, try one of these:
sudo sysctl -w kernel.unprivileged_userns_clone=1
Or set setuid on the sandbox binary:
sudo chown root: ~/.cursor-server/bin/linux-x64/.../cursorsandbox
sudo chmod 4755 ~/.cursor-server/bin/linux-x64/.../cursorsandbox
More details in this thread: "Activating Extensions" message, extensions don't work -- solution
If you don’t have sudo access, there’s no user-side workaround right now. I’ve passed this to the team, and your report helps raise the priority.