systemd PrivateIPC setting errors "Operation not permitted"

Setting [PrivateIPC=true](https://www.freedesktop.org/software/systemd/man/latest/systemd.exec.html#PrivateIPC=) in a user systemd service causes following to be printed in journal:

(myservice): myservice.service: PrivateIPC=yes is configured, but IPC namespace setup failed, ignoring: Operation not permitted
(myservice): myservice.service: Failed to set up mount namespacing: /dev/mqueue: Operation not permitted
(myservice): myservice.service: Failed at step NAMESPACE spawning myservice: Operation not permitted

The linked documentation above mentions > This option is only available for system services, or for services running in per-user instances of the service manager in which case PrivateUsers= is implicitly enabled (requires unprivileged user namespaces support to be enabled in the kernel via the "kernel.unprivileged_userns_clone=" sysctl)

$ cat /proc/sys/kernel/unprivileged_userns_clone
1

Also other settings with the same caveat, e.g. [PrivateNetwork](https://www.freedesktop.org/software/systemd/man/latest/systemd.exec.html#PrivateNetwork=) , seem to work. What gives - why are the errors above shown?