Wi-Fi : listed in lsusb, ath9K installed, jxself linux-libre installed

You say you have a working WiFi card, the kernel I compiled, the ath9k driver (which which actually comes with the kernel) and you keep rebooting. But you don't seem to say what your actual problem is. When you have this working stuff (since you said you have a working WiFi), *why* are you rebooting? What's going on?

Sorry, I'll clarify :
When I say I'm rebooting (by choice), it's only meaning that I tried this option, since it solves issues sometimes.

When I say it's a working card, I mean it's recognized by lsusb, and it worked up until now (my SSD died, I had to reinstall on a new disk).

My problem is that I can create the network interface in Mate (SSID and passphrase, checked both), but when I try to connect to it, it doesn't get connected.

I checked them from another machine, all caps or not, etc.
I also have the document from my ISP where they're printed/written.

Yes, personal, not pro.

Nothing really :
most of my data was on an encrypted HDD.
The OS was on the SSD, along some configuration files and .org files (oh well...).

So I picked an old HDD from an older machine which I was using as an external drive, erased it and made a new install.

Ah, maybe this matters:
this time only /boot is left unencrypted.
But since I input the passphrase and can boot, I doubt it's a problem.

I can try to reinstall the OS, it can solve things in my experience.

Since you mention lsusb I wonder if you're talking of ath9k_htc and not ath9k. ath9k_htc is a USB WiFi. If it's the USB WiFi one please check if the package open-ath9k-htc-firmware is installed.

It is indeed ath9K_htc.

I reinstalled the whole OS, and now the proper Wi-Fi network is already listed. But after inputting the passphrase, I still can't access the network via Wi-Fi.
I'll reinstall the linux-libre kernel now, and see how it goes.

Btw, for your linux-libre tutorial:
"gpg --with-fingerprint gpg.asc" was incomplete in my case.
But "gpg --show-keys gpg.asc" did the trick (the fingerprint shows, although without spacing).
And thank you for your repo :)

EDIT:
I changed the Wi-Fi channel on my modem. My other machine is connected via Wi-Fi, but the current one, still not :/
I double checked the passphrase, I really don't know what to do.

ANSWER
It was a bug.
https://blog.sombex.com/2017/12/fix-cant-connect-to-wifi-issue-in-debian-9-and-other-debian-based-linux.html

1. Open this file:
sudo gedit /etc/NetworkManager/NetworkManager.conf

2. add these 2 lines and save:
[device]
wifi.scan-rand-mac-address=no

3. Restart Network Manager with this line:
sudo systemctl restart NetworkManager

4. Worst case, restart the machine.