The change in the monitor system to query servers multiple times seems to have a positive effect. The black hole which was called Germany sees a significant increase in active servers 
Almost all available servers are now marked active
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My server have reverse effect on test server. Amsterdam server sometimes have some drops like Newark. Third server improves the situation a bit.
I did a deeper dive in my own data of NTP server routes and it seems that Hetzner is no longer peering with Telia for a number of destinations. As I expect their datacenters to contain many NTP servers due to their price and bandwidth, this may also be the cause of the increase in available NTP servers in Germany.
I have also some positive effects before Hetzner changed their routes.
OT:
Hetzner is talking to German Telekom to get a Tier-1 peering which should be established April 1st (no joke).