Would just putting "test" in the subject and body not mean an empty recipient list though?

Available as updates-testing package.

I will do my best to provide this, if you tell me how.

I was more interested in the request body on activesync/http protocol level.

This has been submitted to the OBS.

Normal text (just tried by just putting "Test" as subject and "Test" as content).

The error tells us that the activesync request body is missing or syncroton faild to parse it.
So, to track this down we'd need to know what exactly was sent by the device.

This seems to occur with https://git.kolab.org/T1023 already applied.