Few days ago, I upgraded to latest version of Ubuntu (20.04 LTS). After freshly installing a new system I discovered, that the computer could connect to IPv6 addresses only after a huge timeout. DNS servers are configured properly, ping command finds a correct IPv6 address, but show 100% are lost in first few minutes, then it will magically start working. Sites for testing IPv6 shows that my IPv6 is not working.
The problem is probably caused by a specific combination of both hardware and software. The same version of Ubuntu works on the other devices, and all the other Linux distributions I tried (including Ubuntu 18.04) works without any issues on the same hardware too. I'm currently using Kubuntu 20.04, but the same issue is in default Ubuntu too. It even can be replicated in live USB sessions.
lspci command shows I have RTL8111/8168/8411 PCI Express Gigabit Ethernet Controller (rev 0c)
Obviously, I disabled the IPv6 completely and it solved all connectivity issues I was having. But it still bothers me that the IPv6 is not working for me on the latest version of Ubuntu. What could be causing that? Do you have any idea how to fix it?