Hi Kovidgoyal,
Thank you for your help. With your hints I finally managed to get it work.
Strangly enought when I opened sites links in IExplorer and then tried to download recipes in Calibre it worked ! I did the same trick with couple of other sites with which certification verification error was triggered.
Perfect! I can once again enjoy rss content downloaded on my kindle.
Thanks a lot for your help and spot-on advices - much appreciated
Kind regards.