Thank you, that's almost the same what I also found already, after tinkering the URL in the browser address bar. https://github.com/moxiecode/plupload/archive/v1.5.8.zip
However, the js folder in the 1.5.8 download does not contain the file plupload.full.min.js mentioned in the plupload.txt. So I desperately imported all the files in the js folder into the database Resources Files section.
Where I now have got so far is the Bildr web UI working in general, but any upload of pictures fails. In “Upload Step 2 - Select Image files” the UI presents a “You browser doesn't have Flash, Silverlight, HTML5, or HTML4 support.“. Same time, the server console throws “HTTP Web Server: Couldn't find design note - XSPUploader.js [/bildr/bildr.nsf/XSPUploader.js] CN=Administrator/O=Acme”. So far, I was not able to locate a design element with this name in the bildr database.