GeoServer 1.6.5 problem with WMS GetFeatureInfo on OpenLayers client

I just installed v.1.6.5 two days ago (Windows self-installer on Vista, GeoServer built-in Jetty servlet engine), and everything seems to work OK, except...

If I look at the sample data (or my own data) in the GeoServer OpenLayers client, it looks fine.

But when I click on a feature e.g. in the topp:states layer to display the feature information, I just get a big blob of what looks like an ASCII dump of a binary image (in Firefox), or a few characters in IE "�PNG ".  The same thing happens with all the layers I've tried.

I've had a look at the Javascript for the request, but it looks OK to me.  But I'm no expert!

I'm hoping to use the OL client for a demonstration project next week, so does anybody know what the problem is?  Or should I just go back to v.1.6.3, the last stable version I used up until this week?  

Thanks for any help.

Chris