call_user_func_array() expects parameter 1 to be a valid callback, function 'access_denied_node_form' not found
We have had this problem before in Drupal 6. It can happen if a content type is deleted, but, for whatever reason, nodes of that type still exist.

In this case, it would suggest that you have (or had) a content type called "access_denied" and this is no longer there.

First, note down the node ID of the node you're trying to edit. Then, find the node ID in the {node} table in the database, and find what node type it is.

Then, look in {node_type} in the database to see if your node type is listed there.

If you're missing a node type, you will need to restore the node type before you can edit that node. For example, if you uninstalled a contrib module that provided a particular content type, and that module didn't uninstall cleanly, you might have this issue, so you'd need to reinstall that module to fix things up.

There isn't any way that I know of to find the base module associated with a particular node type, unless it's obvious (the book module provides a node type of book, or similar), but if you are really stuck, you can always delete the node by visiting /node/123/delete - after all, if the base module is no longer present, the node itself probably isn't very useful to you.