As I am also a Netcup customer, i know that their default DNS Server for VPS/Root Servers are usually quite reliable, but my old Windows Server occasionally has hiccups in the evening hours with internet connectivity.
If using a different DNS Server does not solve the issue from reappearing, do note down the times of when you’re having troublr - perchance we can combine our data to make a proper report for their support team to look into!