Here's a detailed description of my problem.

First, my setup:
OS: Manjaro 21.1.0 Pahvo
Kernel: x86_64 Linux 5.10.52-1-MANJARO
DE: KDE 5.84.0 / Plasma 5.22.3

I'm using an ASUS TP300LA laptop with onboard bluetooth.

Here are the packages I have installed

bluez
bluez-hid2hci
bluez-libs
bluez-utils
bluez-tools
bluez-plugins
bluez-qt
bluez-cups
pulseaudio-bluetooth
manjaro-bluetooth
bluedevil

My issue: I'm trying to connect to my wireless headphones, named ATH-SR5BT. I have no issues using them form my phone, but my laptop refuses to connect. My phone, by the way, is completely turned off as to not interfere with the process. I'm not near any other device that has ever been connected to my headphones.

Behaviour

When clicking the KDE Plasma bluetooth widget, insted of it opening a list of bluetooth devices as it used to, it just opens this small window SCREENSHOT 1.

When clicking the gear in order to access bluetooth configuration, the list of bluetooth devices is displayed correctly SCREENSHOT 2. As you can see, the device is displayed correctly. When I click the button to connect the device, the device appears as connected, but the following error message is displayed. "Resource temporarily unavailable" SCREENSHOT 3. The device then disconnects automatically a few seconds later SCREENSHOT 4. The headphones never make the sound they usually make when a device is connected to them via bluetooth.

Running bluedevil-wizard, the device doesn't appear on the list of available devices. SCREENSHOT 5.

Trying to connect vie bluetoothctl, the following error shows:

[[bluetooth]# connect C4:45:67:09:A2:5B 
Attempting to connect to C4:45:67:09:A2:5B
[CHG] Device C4:45:67:09:A2:5B Connected: yes
Failed to connect: org.bluez.Error.Failed
[CHG] Device C4:45:67:09:A2:5B Connected: no

Running sudo systemctl status bluetooth.service outputs the following:

● bluetooth.service - Bluetooth service
     Loaded: loaded (/usr/lib/systemd/system/bluetooth.service; enabled; vendor preset: disabled)
     Active: active (running) since Sat 2021-08-21 13:09:38 CEST; 22min ago
       Docs: man:bluetoothd(8)
   Main PID: 669 (bluetoothd)
     Status: "Running"
      Tasks: 1 (limit: 14207)
     Memory: 2.3M
        CPU: 70ms
     CGroup: /system.slice/bluetooth.service
             └─669 /usr/lib/bluetooth/bluetoothd

Aug 21 13:10:19 joaquin-tp300la bluetoothd[669]: profiles/audio/avdtp.c:avdtp_connect_cb() connect to C4:45:67:09:A2:5B: Connection refused (111)
Aug 21 13:10:22 joaquin-tp300la bluetoothd[669]: profiles/audio/avdtp.c:avdtp_connect_cb() connect to C4:45:67:09:A2:5B: Connection refused (111)
Aug 21 13:11:44 joaquin-tp300la bluetoothd[669]: src/profile.c:ext_connect() Headset Voice gateway failed connect to C4:45:67:09:A2:5B: Connection refused (111)
Aug 21 13:11:45 joaquin-tp300la bluetoothd[669]: profiles/audio/avdtp.c:avdtp_connect_cb() connect to C4:45:67:09:A2:5B: Connection refused (111)
Aug 21 13:11:48 joaquin-tp300la bluetoothd[669]: profiles/audio/avdtp.c:avdtp_connect_cb() connect to C4:45:67:09:A2:5B: Connection refused (111)
Aug 21 13:23:59 joaquin-tp300la bluetoothd[669]: src/profile.c:record_cb() Unable to get Headset Voice gateway SDP record: Host is down
Aug 21 13:24:13 joaquin-tp300la bluetoothd[669]: src/profile.c:ext_connect() Headset Voice gateway failed connect to C4:45:67:09:A2:5B: Connection refused (111)
Aug 21 13:24:14 joaquin-tp300la bluetoothd[669]: profiles/audio/avdtp.c:avdtp_connect_cb() connect to C4:45:67:09:A2:5B: Connection refused (111)
Aug 21 13:24:17 joaquin-tp300la bluetoothd[669]: profiles/audio/avdtp.c:avdtp_connect_cb() connect to C4:45:67:09:A2:5B: Connection refused (111)
Aug 21 13:30:59 joaquin-tp300la bluetoothd[669]: src/profile.c:record_cb() Unable to get Headset Voice gateway SDP record: Host is down

I'm at a complete loss as to how to fix this. Any help is appreciated.

EDIT: The wiki has this to say about it.

Device connects, then disconnects after a few moments If you see messages like the following in the journal, and your device fails to connect or disconnects shortly after connecting: bluetoothd: Unable to get connect data for Headset Voice gateway: getpeername: Transport endpoint is not connected (107) bluetoothd: connect error: Connection refused (111) This may be because you have already paired the device with another operating system using the same bluetooth adapter (e.g., dual-booting). Some devices cannot handle multiple pairings associated with the same MAC address (i.e., bluetooth adapters). Follow instructions on #Dual boot pairing for solving this issue.

I don't have dual booting, or anything else installed on my computer.

EDIT 2: I Solved it by following this thread, using bluetoothctl to remove the device and pairing it again