Custom fields: Ajax not working

I just upgraded to WP 2.9.1 and almost everything works great. However, one thing is bothering me and I would really appreciate any help.

The nice little flashing Ajax thingy which should appear when you enter a custom field and press Add is not working for me anymore. It works great when deleting custom fields but not when adding. If I add a new field, pressing Add and then hit f5 for a refresh, the custom fields is there and working.

I have no plugins activated and an empty functions.php BUT if I switch theme to e.g Classic, the problem disappears. So, it has something to do with my theme but I have no idea what it could be. Any suggestions?