Lookup by title doesn't work

Well, it's related to the same field but perhaps another problem.
I created a content-type with a required amazon-field using "Lookup by title" as input-type. Whenever I try to create a node, this node cannot be saved - Drupal will simply show an error-mark around the field, delete its content and say no more about it.

System
OS: Debian Linux unstable with custom 2.6.28.2-kernel
DB: MySQL 5.0.75-1 (Debian standard)
Apache: 2.2.11-2 (Debian standard)
PHP: 5.2.6 (Debian standard)
Drupal: 6.10
Amazon API: 6.x-1.0-beta5
CCK: 6.x-2.1
Quite a couple of other modules (views 6.x-2.3, currency, money, date, read more tweak, imce, wysiwyg, etc etc etc) but I don't think that they interfere. I am using a custom theme (all the node- and page-information from the default theme is printed someplace on the page) and the node is accessed directly and fully (no teaser, no VIEWS-interaction, just node/x-access).
There are no entries in my Drupal log-files.
Amazon-API-settings: Using default settings or custom settings changes nothing.

Steps to reproduce error

• Create content-type containing required amazon-textfield with "Lookup by title".
• Try to create a node with a correctly filled out "Lookup by title"-filed (I tried the title and various other information in several different formats including "all lowercase", "All uppercase", "Spelling copied from Amazon", "Escaped", "Quoted", "ASIN", ISBN-13, etc)
• Click on Preview or Save - and see the same page again, only with an empty amazon-field and an error-flag around it.