Short version

If you change the text of the form submit button to something with spaces such as "Save Form" you break Node Import. The following code is similar to what is in my module.

function mymodule_form_alter(&$form, $form_state, $form_id) {
    switch ($form_id) {
        case 'contenttype_node_form':
          form['buttons']['submit']['#value'] = 'Save Form';
        break;
    }
}

Now the long version

This took me hours to figure out, some of my guesses were that I'd created an invalid bit of CCK, or my data was invalid and I trawlled though my 1000s of rows of CSV to try and find something wrong, I stripped back the content type to simple text fields, I was running a slightly old version of node import so I tried to update that, I started to look at the PHP version, and I could go on.

I took a break and when I came back it dawned on me that perhaps the cause was another module in the system that was conflicting with node import, ah, very quickly after having that thought, lets say about 5 seconds, I though, oh man only my module is changing form elements for these content types. I turned off the module, imported and it worked! I couldn't leave it at that though, I re-enabled my module.

I then tracked down the line in my module that changed anything on that form and it turned out that it was changing the text of the submit button to "Save Form".

So I commented out that line of code and the node import was successful!

I haven't tracked this into the Node Import code, hopefully the devs will know what this will be able to fix it much more quickly than me.

Drupal version: 6.19 (6.x)
PHP version: 5.2.10-2ubuntu6.10