After better examining my logs, I managed to solve it. I found the following error in /var/log/syslog:
Jan 19 14:27:06 alan bluetoothd[475]: a2dp-source profile connect failed for 78:F8:82:48:55:C3: Protocol not available A2DP is the "Advanced Audio Distribution Profile", and I found a related debian wiki page: https://wiki.debian.org/BluetoothUser/a2dp I installed the required packages listed in that page, and now I can connect to my smartphone via bluetooth.