Well, not really. First to be clear, my intention was to only sync my calender an not contacts, because I don’t have any adressbook maintained remotely.
After reading yours and the reply at the Simple Mobile Tools forum (link see my first post), I deleted the DAVx5 and Android Account hoping that my recent contacts might appear again. “Withtout any Android account” was the actual state before setting up DAVx5 sync, besides maybe a built-in, implicit one. I don’t know if this exists under the hood, but in the Settings there was no account shown before. I’m sure about that.
Additionally the statement in the Simple Contacts Pro forum, that the contacts app is agnostic to any account and kind of takes what it gets/finds on a device in terms of contacts, made me hope that the contacts might be seen again after account deletion.
Putting the provided information together, I tend to say, that DAVx5 was at least involved in the deletion of my contacts. Unfortunately even after account removal there were just the few contacts left, that I recreated after losing the whole address book on my Android device.