Cannot add a tunnel to /etc/network/interfaces

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Note:

I already checked this question and this question. The solution of the former is not relevant to my problem and the latter does not have any answers.

I have a basic VPS from aeza.net. When I ping with IPv4 it is pretty stable and the mdev is less than 10ms, however, IPv6 pings are pretty unstable and have huge spikes. I decided to give tunnelbroker.net a try. I registered for a tunnel and I followed the directions for Debian/Ubuntu given on the website. According to the instructions, I had to edit /etc/network/interfaces and add another interface. Here is how my interfaces file looks like now:

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
auto ens3
iface ens3 inet static
    address   109.120.xxx.xxx
    netmask   255.255.255.255
    gateway   10.0.0.1
    hwaddress ether 52:54:00:17:xx:xx
    dns-nameservers 1.1.1.1 8.8.8.8
iface ens3 inet6 static
    address   2a0b:4140:xxxx::xxxx
    netmask   48
    gateway   2a0b:4140:xxxx::xxxx
    dns-nameservers 1.1.1.1 8.8.8.8

auto he-ipv6
iface he-ipv6 inet6 v4tunnel
    address 2001:470:27:xxxx::xxxx
    netmask 64
    endpoint 216.66.xxx.xxx
    local 109.120.xxx.xxx
    ttl 255
    gateway 2001:470:27:xxxx::xxxx

Then I proceeded restarting networking.service, but it couldn't restart successfuly and gave me the message:

Job for networking.service failed because the control process exited with error code.
See "systemctl status networking.service" and "journalctl -xeu networking.service" for details.

Upon looking at systemctl status networking.service I see the following:

× networking.service - Raise network interfaces
     Loaded: loaded (/lib/systemd/system/networking.service; enabled; vendor preset: enabled)
     Active: failed (Result: exit-code) since Thu 2024-06-06 16:18:54 CEST; 43s ago
       Docs: man:interfaces(5)
    Process: 8524 ExecStart=/sbin/ifup -a --read-environment (code=exited, status=1/FAILURE)
   Main PID: 8524 (code=exited, status=1/FAILURE)
        CPU: 16ms

Jun 06 16:18:54 systemd[1]: Starting Raise network interfaces...
Jun 06 16:18:54 ifup[8535]: add tunnel "sit0" failed: No buffer space available
Jun 06 16:18:54 ifup[8524]: ifup: failed to bring up he-ipv6
Jun 06 16:18:54 systemd[1]: networking.service: Main process exited, code=exited, status=1/FAILURE
Jun 06 16:18:54 systemd[1]: networking.service: Failed with result 'exit-code'.
Jun 06 16:18:54 systemd[1]: Failed to start Raise network interfaces.

I have had tunnels from hurricane electric before and they worked flawlessly when following the same instructions, albeit on different servers from different providers. What am I doing wrong here?