it sounds like a temporary error from the server side. Seems that your account is sometimes not reachable, or the server is responding too slow. Or maybe you’re in a different network (maybe a home WiFi) that has additional firewall restrictions that prevent the connection between DAVdroid and your server.
The vibration is a general setting of your phone - maybe you can change it in Android Settings / Apps. DAVdroid doesn’t have any special settings for notifications. Android manageds these notifications.
Hope this helps a bit…