ConnectIQ iOS SDK does not open my app when fetching device list [Swift]

SyntaxError: JSON Parse error: Unexpected identifier "I"
I am trying to set up the ConnectIQ iOS SDK.

I've followed the documentation:
Added ConnectIQ.framework
Setup my URL Scheme ("frederik-url-scheme")
—> I confirmed that this works: when I type "frederik-url-scheme://" in Safari, my app will be opened!

Added LSApplicationQueriesSchemes to my plist

Then I'm calling
ConnectIQ.sharedInstance().initialize(withUrlScheme: "frederik-url-scheme", uiOverrideDelegate: nil)

And then, when I press the "Connect" Button this code is executed:

Then, the Garmin app opens.
And then nothing happens.

I would expect the same behavior like in the example app, which is that the Garmin app immediately returns to my app by calling my URL scheme. But this does not happen.

I've found similar problem descriptions from a few years ago, but those discussions didn't help me so far:

So in the end I have no idea what's going on there. What am I missing?