Annother problem I noticed in this context: The SVG file requires the '.svg' extension, otherwise the warning 'Unknown image format' message is printed out and the SVG is missed in the pdf. Again: With PNG files the filename extension are not needed!
At the moment Prince does not try to guess whether a file is SVG or not. If it is a HTTP URL, then it will check the content-type header returned by the server, and if it is a local file it will check if it has an .svg extension.
For other image types, it will try loading it in different ways, due to the high number of misidentified image files, eg. PNG images saved with .jpeg extension.
Are you using a different extension for your SVG images?
Thereby the CGI-Program exports the SVG from a database. This is why our SVG filesnames don't have any extension currently. It would be extensive to change that. Perhaps you consider to let Prince also do a SVG check. That would be very helpful for us!
But it is originally the separator between url component path and query string which is never escaped. The resulting filename comes from the download with wget. We had no problems with Prince 9 concerning that.
The new Prince behaviour for URL parsing is more correct according to the specification, and also consistent with what web browsers do, so it is generally not possible to interpret the unescaped ? character without breaking other behaviour.