It took one day to lower the priority of IPv6 on one of the servers at Hetzner.de, as the IPv6 network was unstable, some hosts were periodically unavailable by IPv6, delays appeared, etc.
To decrease the priority, just open the /etc/gai.conf file in the text editor (for example, nano, in which Ctrl+X to exit, and y/n to save or cancel changes):
Find the line there:
#precedence ::ffff:0:0/96 100
And uncomment it or add:
precedence ::ffff:0:0/96 100