D6: Inserting Lightbox styles in CKeditor broken

When trying to Insert a Lightbox style into CKeditor, I am getting mal-formed HTML with nested quotes (see bold text and screenshot):

<a rel="lightbox[group1][__description__<br /><br /><a href="/sites/mydomain.co.uk/files/myimage.jpg" target="_blank" id="lightbox2-download-link-text">Download Original</a>]" href="/sites/mydomain.co.uk/files/myimage.jpg"><img src="/sites/mydomain.co.uk/files/imagecache/image198/myimage.jpg" alt="" class="imagecache-image198" /></a>

I can insert basic styles into CKeditor (eg. Automatic, Original Image, Link to File, and Imagecache images). In CKeditor I've tried disabling (a) Security>Security filters "HTML filter" (b) Advanced content filter (c) Advanced options>Custom JavaScript configuration "config.allowedContent = true;". I've cleared caches.

I'm using Insert 6.x-1.2, CKeditor 6.x-1.15 (with CK editor 4.4.7), Lightbox2 6.x-1.11 and ImageField 6.x-3.11 (all latest versions as of 16 Apr 2015).

Any suggestions?