In the iOS Calendar App there is 2 'Birthday' calendars bellow the 'other' section when I tap the 'Calendars' button at the bottom of the screen and there is no option to remove them (neither in the app nor in the iPhone Settings). The only available option is to uncheck them.
Strangely only one contains birthdays from my Contacts Adresse Book while the other one is empty.
I only use iCloud for Contacts, Calendar and Reminders accounts (my other Google and Exchange accounts are set on Mail only).
I tried many solutions found around the web including ones given by Apple (turning off Calendar, Contacts and Reminders in iCloud settings, and then turning back on after a while), I tried solutions given by users (closing apps, deactivate iCloud, rebooting the phone) and I tried restoring my iPhone.
Note: I have an iPhone 7 Plus with iOS 11 installed and an early-2015 MacBookAir. I'm also using iCloud since the beginning of its existence.
Thank you in advance for your help!
As asked in the comments here is somes screenshots for clarity (feel free to ask more, i'll upload as soon as possible).
Sorry for the French language in second screenshot (as well as for the cropped aspect, I don't have access to a computer for now). Note that I tried to uncheck the "show birthday calendar" in iCloud Calendar Settings and when I came back later it was still enabled (I didn't forget to hit the save button, however and I tried several times).