This is just to rule something out rather than a definite answer...
Sometimes these types of errors come up when an incorrect version of PHP is running the cronjob. I've had multiple versions of PHP running on a server, with the default being an older version of PHP incompatible with eZ Publish 4.0.1.
To get the cronjob to run correctly, the correct version of PHP to run must be specified in the command line used to run the cronjob script.
If this is the problem, then some of the information here may help: http://ez.no/developer/articles/how_to_import_and_export_rss_feeds/rss_import_ez_publish_cronjob_configuration
It's only a warning triggered when an item that has already been imported is encountered, so apart from the annoying output the code should function correctly.
The quickest fix is to replace "$linkURL" with "$link->textContent||$guid->textContent".
Actually there is a bit of refactoring that could occur in this section of code.
