RPi 2 GPIO UART can send data to Teensy, but can't receive

Question

I am trying to send data between RPi 2 (model B) and Teensy 3.2 using the GPIO on the Pi (GPIO 14 and 15) and Serial2 on the Teensy (pins 9 and 10).

The problem is, I can only send data from the Pi to the Teensy, but not the other way round.

Python:

import serial
ser = serial.Serial("/dev/ttyAMA0", 9600, timeout=3)
print(ser.is_open)  # True
while True:
    data = ser.readline()
    print("Data: {}".format(data))

Teensy:

void setup() {
    Serial2.begin(9600);
    Serial2.setTimeout(500);
}
void loop() {
    Serial2.println("hello");
    delay(1);
}

The Python code always ends up in a timeout (reading empty data).

Things I've tried and didn't help:

Code:

• Changing the timeout, and without timeout
• Loop delay (to slow down read/write speed)
• Testing with read() and print() instead of readline() and println()

Pi:

• Restart (of course)
• Disable and re-enable the UART via raspi-config ("would you like the serial port hardware to be enabled")
• Disable "login shell over serial" (again raspi-config), I read somewhere it might help
• Using sudo

But nothing helped.

I also checked this: UART Send/Receive not working but with no success.

• When "login shell over serial" is enabled, sudo is required in order to run the Python code (access the port). When it is disabled, it isn't required.

• When I check ls -l /dev/* output, it says "*serial0 -> ttyAMA0, and one line above serial. That's all it says about serial. (Not quite sure if that even helps)

• The file /boot/config.txt has enable_uart=1

I checked the wiring and everything seems to be fine, so I assume it has something to do with the Pi software.

I thought maybe the RX pin is already used by something else, but I couldn't find if, and how I can check it.

What else can I try? Is there something I am missing? Thanks in advance for any help!