In case anyone else has the same problem, I saw this error when I upgraded my WordPress 3.0.1 installation to 3.1 using the automatic upgrade.

The dashboard worked fine, but when I tried to look at the site itself the page wouldn’t load in IE 8 or Firefox 3.6.  With Google Chrome I get the following message (and a similar one in Firefox):

Before I made the upgrade I installed a TEST instance of WordPress on the site with a clean copy of WordPress 3.0.1.  I performed the upgrade from the Dashboard and then just uploaded my Kajabity Theme.  I used this to check the upgrade would be OK – and the Dashboard worked fine – shame I forgot to check the public facing site still worked…

So, having returned the TEST site to it’s pristine form (i.e. back to the default theme) I find that the problem sits square in the lap of the upgraded WordPress.

I checked Google – as you do – and didn’t find anything that helped.  There was only one solution: I used FileZilla to transfer the older version’s files back over the updated ones and hoped for the best.  A backup might have been a good idea.

Still, it worked – the site worked again and, after telling me the database needed “Upgrading” (?!) so did the admin screens.

I re-enabled the plugins – a couple of them complained when I upgraded and I had to re-install Akismet.

And as you can see (by reading this post) it’s all working now.  Good.

I’ll try WordPress again when they appear to have fixed the problem – or I find a good explanation.