Hi,
It can be a case our base images (ran in privileged mode and UDEV on) modify the permission of the ptmx node. Which makes any opening of a pseudo terminal impossible until the permissions are fixed (see how our base images handle udev in the entry script https://github.com/balena-io-library/base-images/blob/master/scripts/assets/entry.sh#L14-L52). Can you please add chmod 666 /dev/pts/ptmx to fix the permission issue and let us know the result?