works with one, not with the other.

I have a number of sites hosted on Mediatemple – I have just installed two new ones – both are virgin installs of 3.9 I have made sure that the same themes and plugins are installed. – One works fine – the other is consistently crashing with out the database update loop – which seems to have been mentioned a thousand times, but there doesn’t seem to be a clear definitive answer as to what causes it, or how to fix it.

I am also routinely seeing things that look like the following…

Fatal error: Undefined class constant 's:17:"xml_base_explicit";b:0;s:8:"xml_lang";s:0:"";}}s:4:"link";a:1:{i:0;a:5:{s:4:"data";s:54:"http://wordpress.org/plugins/contact-form-7/#post-2141";s:7:"attribs";a:0:{}s:8:"xml_base";s:0:"";s:17:"xml_base_explicit";b:0;s:8:"xml_lang";s:0:"";}}s:7:"pubDate";a:1:{i:0;a:5:{s:4:"data";s:31:"Thu, 02 Aug 2007 12:45:03 +0000";s:7:"attribs";a:0:{}s:8:"xml_base";s:0:"";s:17:"xml_base_explicit";b:0;s:8:"xml_lang";s:0:"";}}s:4:"guid";a:1:{i:0;a:5:{s:4:"data";s:34:"2141@http://wordpress.org/plugins/";s:7:"attribs";a:1:{s:0:"";a:1:{s:11:"isPermaLink";s:5:"false";}}s:8:"xml_base";s:0:"";s:17:"xml_base_explicit";b:0;s:8:"xml_lang";s:0:"";}}s:11:"description";a:1:{i:0;a:5:{s:4:"data";s:54:"Just another contact form plugin. Simple but flexible.";s:7:"attribs";a:0:{}s:8:"xml_base";s:0:"";s:17:"xml_base_explicit";b:0;s:8:"xml_lang";s:0:"";}}}s:32:"http://purl.org/dc/elements/1.1/";a:1:{s:7:"creator";a:1:{i:0;a:5:{s:4:"data";s:16:"Takayuki Miyoshi";s:7:"attribs";a:0:{}s:8:"xml_base";s:0:"";s:17:"xml_ in /nfs/c02/h05/mnt/21889/domains/linchpinresources.com/html/wp-includes/wp-db.php on line 1910

Looking at the Source (I am not a php wiz) it looks like a datbase query, but in this instance it appears to be in relation to a plugin for a contact form – I have no such plugin installed or activated…

What gives.