FWIW, I'm also getting the following error on another multisite install (2 sites) and that one does not have WP-Table Reloaded installed as was mentionned above. The error at the top of blog 2's admin pages:
Warning: Invalid argument supplied for foreach() in /path/to/wp-content/plugins/wordpress-seo/inc/wpseo-functions.php on line 407
That's with version 1.2.3 with WP 3.4
M. Penz post right before mine gave me a clue to look int the database and I found something interesting. The "wpseo" option does exist on this 2 site ms network but the value for the blog 2 version is weird. The blog 1 version of the wpseo option contains this serialized array:
But the same option in the blog 2 options table contains this:
... which is a simple text string and not a serialized array which the code probably expects. I tried to copy the value from the blog 1 options table to the blog 2 options table and the error went away for me. Not sure if this can have other adverse effects on how WordPress SEO works but at least the error is gone for me for now.
But this definitely seems like a bug in 1.2.2 and 1.2.3 and one which probably has nothing to do with a conflict with other plugins.