You have been right. Posteo verified that the error is caused by the broken event. After I have removed the event synchronization works now.
Thanks for your support!
I prefer to have all my calendar entries (last 10 years and more) on my mobile device. If I clear the day parameter for past entries in the davdroid settings it works and I get all my entries from owncloud after synchronizing. But if I change this parameter back to e.g. 90 days (for reducing sync time) my entries oder than 90 days disappear after syncing. Is this a wanted behaviour? Is there a possibility to keep the old entries (difference between long-term and normal sync)?
Yes, this is the wanted behaviour and no, there is no option for that.