For comparison, the same images in a ZIP archive can be opened, and XnView shows thumbnails instead of icons. Vintage 2006 presumably means 7-zip 4.x (2004), the last version before 7-zip 9.x (2009). If I disable AddOn/7z.dll (= temporarily renamed to 7z.dll.OLD) XnView unsurprisingly cannot more show the content of *.7z archives, and if I then try to open the archive anyway within XnView it crashes.
Six years later the next stable version 7-zip 15.14 (2015) inspired some security researchers to check 9.20 and 15.14, and they found an issue (German) for UDF files. UDF is a file system for DVDs, DVD images and other purposes, same idea as ISO-files for CDFS and CD images. There is a new 7-zip 16.00 (2016) published some days ago, where this security issue was fixed.
TL;DR: Please create a new working AddOn/7z.dll based on 7-zip version 16.00, ideally also working with XnView 2.35 for W2K. (The 32bit 7-zip 16.00 version is apparently okay for W2K.)