Adding nodes with cck imagefields - error 1: Can't add additional cck imagefield while another image is in queue

This may be a confusing issue as it uses several modules, so I'll take it slow...

Modules:
CCK
CCK fielfield
CCK imagefield
Popups API
Popups: Add and Reference

Node-types:
photo_cck - this node-type allows for uploading an unlimited number of images via the CCK imagefield.
listing - this node-type may reference photo_cck nodes.

Process:
1 - Click "add photo_cck"
2 - The create node form pops up after a little wait.
3 - "browse" to upload the first image.
4 - click "upload" for that image. Image uploads OK.
or...
4b - while image is not yet uploaded, click the button to add another item. If done through the normal create node process, this loads an additional imagefield. However, when done in the popup, it gives the error "
An HTTP error 0 occurred.
/content/js_add_more/photo-cck/field_photo_cck2"