Oh dear. I tried to be clever and implement an SEO optimization – and as a result anyone who found a search engine result leading to my site and clicked on said link would have found a nasty 404 – page not found message.
Luckily I caught on to the problem quite quickly, and I believe I have resolved it, thanks to this post, and thanks to the plugin that you can find here.
Now I can play with things like permalinks and not have to worry too much about nasty collateral effects!

Stop reading, start speaking
Stop translating in your head and start speaking Italian for real with the only audio course that prompt you to speak.
Thanks to Google, I found the solution quickly. Phew!
Oh the joys of being an amateur webmaster.
Please accept my apologies if my tinkering caused problems.