[UPDATE] There was a block on the troublesome pages with php code in it that no longer worked, so I fixed the problem for now by deactivating the block.

After upgrading from 6 to 7rc3 I have a "500 Internal Server Error" on the site front page (http://mydrupalsite.com/) and on the forum front page (http://mydrupalsite.com/forum).

If I go to administration and change the front page, say from /node/40 to node/50, then http://mydrupalsite.com/ remains unavailable but http://mydrupalsite.com/node/40 becomes available while http://mydrupalsite.com/node/50 gives the 500 error.

In the latter case the error message in the server logs would be

[Thu Dec 23 19:33:00 2010] [error] [client 127.0.0.1] PHP Fatal error: Call
to undefined function db_fetch_object() in /var/www/htdocs/drupal/modules/php/php.module(75) : eval()'d code on line 5, referer: http://mydrupalsite.com/node/50

I've rummaged around trying to find suggestions but most of them have to do with the .htaccess file. That doesn't seem to be the problem, at least not any positive directives in the file, since if I get rid of the whole thing the problem remains.

Any thoughts?