Ultimate Facebook Plugin Error in cache.php file

I've installed the ultimate facebook plugin and get the following error only on posts that have comments -

Warning: Illegal offset type in isset or empty in ........../public_html/wp-includes/cache.php on line 351 (and on several other lines in the same file)

I've disabled all plugins and the error still remains when the ultimate facebook plugin is installed.

All of my plugins, theme etc. are up to date and I'm using the daily theme.

Anyone have any ideas on what to do?

Thanks!
Ruth