Error message on automatic upgrade to WP 2.8

I’m getting the following error on trying to upgrade via the “Upgrade Automatically” link:
“Internal Server Error, this is an error with your script, check your > error log for more information.”

I’ve contacted the hosting company several times and the latest reply is as follows:
“The error being reported is a premature end of script headers which usually means the script aborted before completing, what I can confirm is it is not being killed by the server.

[Mon Jun 29 09:45:54 2009] [error] [client 212.103.233.1] Premature end of script headers: /execscriptdir/php.cgi

As it is not a specific error it would have to be debugged from within the code itself, I would suggest contacting the developers to advise as to the best course of action.”

I have an identical set up locally under MAMP which upgraded perfectly, so I know it’s not me.

Any suggestions welcome, please!