Cannot Save Post on platform with out getting an error on line 233 in DB.php

Any one seen this It has only just started

Catchable fatal error: Object of class WP_Error could not be converted to string in /home/--------/public_html/wp-content/db.php on line 233

I have UPGRADED to MU 2.7.1 I was hoping the problem would disappear but Alas it has'nt
I have the latest Multi database 2.7 DP.php file

When ever anyone on the platform tries to post an article this is the result only on the posting side all other data base entry are working as they should

Im going to Back out some plugins no that did not work

this is the part of DB.php that sends the error

Line 226    function set_blog_id($blog_id, $site_id = '') {
		if ( !empty($site_id) )
			$this->siteid = $site_id;

		$old_blog_id = $this->blogid;
		$this->blogid = $blog_id;

Line 233	$this->prefix = $this->base_prefix . $this->blogid . '_';

		foreach ( $this->blog_tables as $table )
			$this->$table = $this->prefix . $table;

		return $old_blog_id;
Line 239	}

Any Ideas or solution here is real important as I have over 2000 Blogs running on the platform