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First, PR that you're going by is TBPR - Tool Bar Page Rank.  This number is not the actual PageRank that Google assigns, which is private and never revealed.  Also, the TBPR is not updated very often.  Google have said they update it "infrequently" - typically only a couple / few times a year. So that drop is not a very good indicator as to what's happening on the site. Beyond that, your real PageRank is not the only thing to consider.  The real assessment should come from reviewing visitor activity - how much traffic comes from organic listings, cross-compared to how many keyword phrases they use to find the site. And that should only be measured when held up against total conversions - whatever calls to action you have.  Because even a drop in organic visits might not be a bad thing if the total conversions increases due to a higher quality visitor base.

In Open site explorer, you can put in a URL for a specific page, and it defaults to showing backlinks for just that particular page, not the whole domain.

akitmane Are the errors you see pointing to pages that no longer exist?  If that's the case, the best fix that I've found with WP is use of the "Simple 301 Redirects" plug-in for WordPress.  It lets you enter the URL of the old page and the URL of a page you want people to go to if they try going to that old URL. If they're pages that never existed, the question becomes - why are people trying to get to pages that never existed? If they're innocent errors, or links from other sites where the person who made the link made a mistake in the URL, you can either use the Simple 301 Redirects plug-in for those specific links, or contact the site owner of the site that created the flawed link. Ultimately, you should have a custom 404 page set up (located in the same folder as the theme you're using for your site's template).  If that's not working, you will need a WordPress developer or consultant to help you figure out why.

PageRank is simple to understand. Your PR went to 1 because either: The highest pagerank of the linking page is now 2 instead of 4 what it used to be The highest pagerank of the linking page is higher than 2 but with excessive outgoing links splitting up PR too thin PR4 or higher link you had is gone or nofollowed You were selling links and got a filter reduction of pagerank to make it less atractive for purchase (unlikely as your PR is low anyway to start with). The highest pagerank page on your site is now a sub page (did you check?) That's all I can think of for now...