Well, the problem is, the servers worked just fine in the past, up to beginning of Feb. 2020 and stopped working all at once, with no changes made to the config for over a year by now.
In addition to that my other servers are syncing to them just fine + they are in active production use with my customers, there are no restrictions which could cause this, even following the recommended config.
I tried this: Connected server to a Vodafone Business Cable static IPv6 subnet, see here:
https://www.ntppool.org/scores/2a02:8106:19::1
Works just fine.
The very same server hooked to a T-Systems (German Telekom IP) at 2003:a:157f:b76d::0
No luck at all.
Can anyone add/test 2003:a:157f:b76d::0 to verify?
I did ntpsweep -l 2003:a:157f:b76d::0 from several locations, works fine.