rpc_pipefs errors

[hujuice]

On TWO working systems (desktop and laptop) I have the same issue.
When I start rpc.idmapd I receive the unclear error:

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make sure DNOTIFY support is enabled

DNOTIFY is enabled.

If I run:

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rpc.idmapd -f -vvv

the error is:

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Unable to inotify_add_watch(/var/lib/nfs/rpc_pipefs//nfs): No such file or directory

If touch the missing file:

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touch /var/lib/nfs/rpc_pipefs/nfs

This service starts.

I am acting like a monkey: I have no idea about the nature of the missing file. It could be a pipe, a link, it could have special permissions.

In this way, the service starts and the NFS shares work, but the problem is NOT resolved.
Now, when I stop rpc.idmapd, I have another error:

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umount: /var/lib/nfs/rpc_pipefs: not mounted.

while the filesystems looks mounted according to mount:

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rpc_pipefs on /var/lib/nfs/rpc_pipefs type rpc_pipefs (rw,relatime)

Something in my /var/lib/nfs/rpc_pipefs is broken.
The problem is identical on two very similar system, so the problem is either a mistake I keep making or a bug.

Is anybody able to suggest what the problem might be?
Regards,
HUjuice