Potential security issue?

I upgraded my wordpress to verion 2.3.1 a few days ago. Right after the upgrade everything was working fine.

Now I get an error on my blog:
“Fatal error: Call to undefined function: add_action() in /web/www/blog/wp-includes/classes.php on line 307”

It seems that a bunch of files have been modified recently so I wonder if this can be due to a security issue.
Here is the list of file that have been touched (I haven’t modified any of them manually):
-rw-r–r– 1 www-data www-data 188 2007-11-05 07:06 blog/index.php
-rw-r–r– 1 www-data www-data 33998 2007-11-08 10:57 blog/wp-includes/general-template.php
-rw-r–r– 1 www-data www-data 11388 2007-11-01 11:40 blog/wp-includes/gettext.php
-rw-r–r– 1 www-data www-data 11798 2007-11-01 11:40 blog/wp-includes/wp-db.php
-rw-r–r– 1 www-data www-data 22847 2007-11-01 11:40 blog/wp-includes/pluggable.php
-rw-r–r– 1 www-data www-data 6208 2007-11-01 11:40 blog/wp-includes/default-filters.php
-rw-r–r– 1 www-data www-data 23730 2007-11-08 08:58 blog/wp-includes/classes.php
-rw-r–r– 1 www-data www-data 110627 2007-11-01 11:40 blog/wp-includes/endix.php
-rw-r–r– 1 www-data www-data 17193 2007-11-08 10:57 blog/wp-includes/class-mail.php
-rw-r–r– 1 www-data www-data 3315 2007-11-01 11:40 blog/wp-admin/admin.php
-rw-r–r– 1 www-data www-data 5311 2007-11-01 11:40 blog/wp-admin/index.php
-rw-r–r– 1 www-data www-data 58079 2007-11-01 11:40 blog/xmlrpc.php
-rw-r–r– 1 www-data www-data 617 2007-11-01 11:43 blog/wp-content/themes/random-image/footer.php

I installed wordpress on 2007-10-29 in the blog/ directory. I have another instance of wordpress running on the same server (in a different directory) but all its files date from the day I updated them. None of the files in the second blog have been modified.

Any help or comment would be wlcome to understand how this happened.

Thanks,
Matthieu