PNP was called using a host in the URL.

Example:

 http://<nagios-server>/pnp4nagios/graph?host=<this-host>

The resulting path to the perfdata directory could be found but it was not possible to retrieve data.

The path is shown in the error message.

The cause for this error cannot be determined without further ado.

Normally PNP will not create a directory without placing data. Maybe the directory was created manually with wrong permissions so PNP has no write permissions.