2 antivirus softwares cannot work on a single machine. Say u have Norton and FSecure. fSecure wil show Norton as a threat and Norton wil show fSecure as threat.
Similarly, when u have 2 SEO plugins activated on the same WP installation, each will try to override the other plugin's setting and thus causing the errors you are currently seeing. I have been using "All-in-one-SEO" and I have been very satisfied so far. Never found a need of another one. Just try deactivating the Yoast, and your errors should disappear.
Hope this helps.
