**TL;DR**: When upgrading from `Bitcoin Core 0.20.1` to `Bitcoin Core 0.21.0`, a downstream application using the HTTP JSON-RPC API started getting HTTP 403 "Forbidden".

Turns out it is because I have `rpcallowip=::/0` in `bitcoin.conf` which used to work but does not anymore.
As a workaround, `rpcallowip=0.0.0.0/0` works.

-----

I run bitcoind in docker container1, and some application using the HTTP JSON-RPC API of bitcoind is in docker container2. They are on the same "docker network".

In `bitcoin.conf` I have:
```
rpcauth=<...>
rpcbind=0.0.0.0:8332
rpcallowip=::/0
```
(bitcoind RPC is not actually exposed to the public internet; it is up to the docker layer and above to ensure this.)

The application in container2 is connecting through HTTP accessing an URL such as `http://user:pass@172.19.1.10:8332/`.

This setup used to work when running `Bitcoin Core 0.20.1` in container1.

It no longer works with `Bitcoin Core 0.21.0`.

-----

If I change my `bitcoin.conf` as below, it starts working again using 0.21:
```
rpcauth=<...>
rpcbind=0.0.0.0:8332
rpcallowip=0.0.0.0/0
```

So `rpcallowip=::/0` used to work but does not anymore; `rpcallowip=0.0.0.0/0` works.

-----

Easy way to test:
`curl --data-binary '{"jsonrpc":"1.0","id":"curltext","method":"getblockchaininfo","params":[]}' -H 'content-type:text/plain;' http://user:pass@172.19.1.10:8332/`
if (args.GetBoolArg("-reindex-chainstate", false)) {
            return InitError(_("Prune mode is incompatible with -reindex-chainstate. Use full -reindex instead."));
        }
    }
    
    if (args.GetBoolArg("-forcednsseed", DEFAULT_FORCEDNSSEED) && !args.GetBoolArg("-dnsseed", DEFAULT_DNSSEED)){
        return InitError(_("Cannot set -forcednsseed to true when setting -dnsseed to false."));
    }
    
    size_t nUserBind = args.GetArgs("-bind").size() + args.GetArgs("-whitebind").size();
    if (nUserBind != 0 && !args.GetBoolArg("-listen", DEFAULT_LISTEN)) {
        return InitError(Untranslated("Cannot set -bind or -whitebind together with -listen=0"));
    }
    
    if (!args.GetBoolArg("-listen", DEFAULT_LISTEN) && args.GetBoolArg("-listenonion", DEFAULT_LISTEN_ONION)) {
        return InitError(Untranslated("Cannot set -listen=0 together with -listenonion=1"));
    }
    
    int nBind = std::max(nUserBind, size_t(1));
    nUserMaxConnections = args.GetIntArg("-maxconnections", DEFAULT_MAX_PEER_CONNECTIONS);
    nMaxConnections = std::max(nUserMaxConnections, 0);
    nFD = RaiseFileDescriptorLimit(nMaxConnections + MIN_CORE_FILEDESCRIPTORS + MAX_ADDNODE_CONNECTIONS + nBind + NUM_FDS_MESSAGE_CAPTURE);
#ifdef USE_POLL
    int fd_max = nFD;
#else
    int fd_max = FD_SETSIZE;
#endif
    
    
    nMaxConnections = std::max(std::min<int>(nMaxConnections, fd_max - nBind - MIN_CORE_FILEDESCRIPTORS - MAX_ADDNODE_CONNECTIONS - NUM_FDS_MESSAGE_CAPTURE), 0);