Once on one server, on all pages where reCAPTCHA was used, they stopped working: contact form, forum registration form, password recovery forms, etc.
At first it turned out that IPv6 stopped working, so the server could not establish a connection with Google, since IPv6 was not used on this server, then I turned it off:
cat /proc/sys/net/ipv6/conf/all/disable_ipv6 1 cat /proc/sys/net/ipv6/conf/eno1/disable_ipv6 0 echo 1 > /proc/sys/net/ipv6/conf/all/disable_ipv6 echo 1 > /proc/sys/net/ipv6/conf/eno1/disable_ipv6
See my article – Disabling IPv6 on Linux
I note that IPv6 was disabled earlier, but since the IPv6 parameters were specified in netplan in other places, IPv6 was activated on the eno1 network interface.
A little later, the DNS server also stopped working at the hosting provider, I checked it with the usual ping command:
To solve this problem, I added some DNS to /etc/resolv.conf and also specified them in the network configuration /etc/netplan/.
After that, the problem with reCAPTCHA was solved.