When troubleshooting in the IPv6 area, it is particularly important to double-check the IP address, subnet mask, and the default route or default gateway for typos. Due to the nature (length and complexity) of IPv6 addresses, it is easily possible to miss or accidentally add a digit.
In Debian Linux you can work with the following commands:
ip -6 addr show - display configured IPv6 address
ip -6 route show - display configured default route
Examples based on an IP address configured in Austria, at the interface eth0
Please pay attention to use the correct subnet (2a03:f80:xx:xxxx::1) and netmask /48
Please pay attention to use the correct default gateway (2a03:f80:xxx::1) Every server locations has its own unique gateway address!
If the configuration seems correct and IPv6 is still not working properly, this could be a general network issue. A test with mtr or ping provides more insights.
Use an address that you can assume will work with a high degree of probability, such as ipv6.google.com:
mtr -6 -r ipv6.google.com
ping -6 ipv6.google.com