Web Bluetooth Notifications NotSupportedError

Hello!
I am trying to connect a [TI SensorTag CC2650](http://processors.wiki.ti.com/images/a/a8/BLE_SensorTag_GATT_Server.pdf) to Web Bluetooth in a Chrome browsers running on Win10.
I have used [this demo](https://googlechrome.github.io/samples/web-bluetooth/characteristic-properties.html) successfully with the following UUID parameters (Humidity Sensor):
Bluetooth Service: f000aa20-0451-4000-b000-000000000000
Bluetooth Characteristic: f000aa21-0451-4000-b000-000000000000
It outputs:
```
> Characteristic UUID: f000aa21-0451-4000-b000-000000000000
> Broadcast: false
> Read: true
> Write w/o response: false
> Write: false
> Notify: true
> Indicate: false
> Signed Write: false
> Queued Write: false
> Writable Auxiliaries: false
```
Everything in its right place so far :)
Here's what I don't understand:
In the [notification example sample](https://googlechrome.github.io/samples/web-bluetooth/notifications.html), using the same parameters as before i get the following Error:
`Argh! NotSupportedError: GATT operation failed for unknown reason.
`

Why is this happening? :(