Azure > App Service > Mobile App > Offline Sync doesn't work on my Android (Xamarin.Forms)

I'm currently struggeling to get Azure > App Service > Mobile App > Offline Sync running. In my current experiment I'm trying to strictly follow the instruction.

So I did as follows:

Iteration 1

• Defined an App Service on Azure (with new DB etc.)
• Download the Quick Start C# backend VS solution
• Download the Quick Start C# backend VS solution
• Download the Quick Start Xamarin.Forms App
• Integrate the Xamarin.Forms PCL and Droid projects into the 1st solution

=> Test: TodoList App works as expected. Cool!

Iteration 2

• Added OFFLINE_SYNC_ENABLED to "Conditional compilation symbols" to the project properties
• Run the app again on my Nexus 5 (Android 6.0 - API 23)

=> Pull async is called... but nothing happens. So my todoItems stored in the Azure DB do not appear.

The only more or less interesting log i found is this line: 02-23 09:39:55.280 I/mono-stdout(25706): Invalid sync operation: The request could not be completed. (Bad Request)

I couldn't recognize any traffic via fiddler.

For me it looks like the templates on azure wont work...

Am I doing something wrong? Do you have any suggestions?

Thanks a lot!