I got tons of errors during my apt-get update informing me that the repository is currently not available.
Can happen, but after 3 days same message I started to think about to change the repository to a closer / new one.
So I looked around my
/etc/apt/sources.list and searched for a way to modify all old repository entries and replace it with the new one.
sed did the trick for me to be honest
sed -i 's/mirror.skylink-datacenter.de/de.archive.ubuntu.com/g' /etc/apt/sources.list
– and I wonder why I do not use it more oft 😉