Site domain being added to links

on the site https://www.scuoladibondage.it I just realized that the one of the last article I wrote is showing an unusual issue.
This is the page: https://www.scuoladibondage.it/shibari-showparty-19-settembre-2015/
At the end of the article there’s a link to another page within the blog. I added the link using the usual button in the editor and selecting the page to link in the popup menu. I never thought it could be a problem since I used the same method many other times before and it worked.

Now the system shows a link with the site domain put just before the link, so that it all results in a “page not found” error.
This is the link it should point to http:/www.scuoladibondage.it/shibari-showparty-19-settembre

But this is the actual link that is added to the page https://www.scuoladibondage.it/”http:/www.scuoladibondage.it/shibari-showparty-19-settembre/”
It is like if the system now consider the link as a “relative” link, rather than an absolute one.
For sure this is not happening when adding the link, but rather when saving the page (if I check the text editor before saving, the link is just fine)
Infact I realized that when I add the link everything works fine, but the problem arises when I save the page.
I checked the text editor.
When I add the link, the code looks like this:

But as soon as I save it, it turns into

so the backslash seems to be added for no reason

and if I just keep re-saving the page without any further change the link becomes:

and so on….

I tried disabling all plugins and tried re-adding the link, but the problem is still showing.
Is there a way to fix this?

All the links I added in the past worked fine.

As you can see the page also shows another problem, ie, since the italian language uses a lot of ‘, they get turned into \’ each time the page is saved…
maybe the two issues are related? How can I fix the \’ problem?