It appears that Flickr changed something a while ago that broke compatibility with the built-in Flickr plugin in older versions of LR: Flickr: The Help Forum: [investigating] It's back. Flickr Upload Via Lightroom not working, semicolon expected. You've got a few options:
- Upgrade to the new version of LR.
- Use this third-party plugin: Jeffrey Friedl's Blog » Jeffrey’s “Export to Flickr” Lightroom Plugin. The author has an excellent reputation, with many LR plugins (he helped design LR's publishing architecture), which he supports better than Adobe supports their plugins.
- The thread above in the Flickr help forum also references a free Flickr plugin that appears to work for a number of people in that thread. But its author is unknown -- I recommend being conservative and using one of the previous two options.
It's unfortunate that Flickr broke compatibility with old software, but that's one of many symptoms of Yahoo's descent into the death spiral -- Flickr just isn't a reliable service anymore.