Why is it that pgp keyservers do not respond when I request keys?

Question

I tried the Linux command line (bash 4.4.23 on Arch Linux kernel 4.18.16) with various key servers:

$ gpg -v --keyserver subkeys.pgp.net --recv-keys F434104235DA97EB
gpg: using character set 'utf-8'
gpg: keyserver receive failed: Server indicated a failure
$ gpg -v --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys F434104235DA97EB
gpg: using character set 'utf-8'
gpg: keyserver receive failed: Server indicated a failure
$ gpg --debug-level 9 --keyserver  hkp://keyserver.ubuntu.com:80 --recv-keys F434104235DA97EB
gpg: enabled debug flags: packet mpi crypto filter iobuf memory cache memstat trust ipc clock lookup extprog
gpg: DBG: [not enabled in the source] start
gpg: DBG: chan_3 <- # Home: /home/user/.gnupg
gpg: DBG: chan_3 <- # Config: /home/user/.gnupg/dirmngr.conf
gpg: DBG: chan_3 <- OK Dirmngr 2.2.11 at your service
gpg: DBG: connection to the dirmngr established
gpg: DBG: chan_3 -> GETINFO version
gpg: DBG: chan_3 <- D 2.2.11
gpg: DBG: chan_3 <- OK
gpg: DBG: chan_3 -> KEYSERVER --clear hkp://keyserver.ubuntu.com:80
gpg: DBG: chan_3 <- OK
gpg: DBG: chan_3 -> KS_GET -- 0xF434104235DA97EB
gpg: DBG: chan_3 <- ERR 167772379 Server indicated a failure <Dirmngr>
gpg: keyserver receive failed: Server indicated a failure
gpg: DBG: chan_3 -> BYE
gpg: DBG: [not enabled in the source] stop
gpg: keydb: handles=0 locks=0 parse=0 get=0
gpg:        build=0 update=0 insert=0 delete=0
gpg:        reset=0 found=0 not=0 cache=0 not=0
gpg: kid_not_found_cache: count=0 peak=0 flushes=0
gpg: sig_cache: total=0 cached=0 good=0 bad=0
gpg: random usage: poolsize=600 mixed=0 polls=0/0 added=0/0
              outmix=0 getlvl1=0/0 getlvl2=0/0
gpg: rndjent stat: collector=0x0000000000000000 calls=0 bytes=0
gpg: secmem usage: 0/32768 bytes in 0 blocks
$ 

I tried the web interface of https://pgp.surfnet.nl/. It responds

Service Unavailable

The server is temporarily unable to service your request due to maintenance downtime or capacity problems. Please try again later.

I tried the web interface of https://pgp.mit.edu. It responds:

Proxy Error

The proxy server received an invalid response from an upstream server. The proxy server could not handle the request GET /pks/lookup.

Reason: Error reading from remote server

I tried the web interface at https://pgp.key-server.io Reply is:

Hint! Double-check if the keyserver is up and running at the expected address:port (127.0.0.1:11369).
cURL error 52: Empty reply from server
#0 /var/www/pgp.key-server.io/vendor/guzzlehttp/guzzle/src/RequestFsm.php(103): GuzzleHttp\Exception\RequestException::wrapException(Object(GuzzleHttp\Message\Request), Object(GuzzleHttp\Ring\Exception\ConnectException))
#1 /var/www/pgp.key-server.io/vendor/guzzlehttp/guzzle/src/RequestFsm.php(132): GuzzleHttp\RequestFsm->__invoke(Object(GuzzleHttp\Transaction))
#2 /var/www/pgp.key-server.io/vendor/react/promise/src/FulfilledPromise.php(25): GuzzleHttp\RequestFsm->GuzzleHttp\{closure}(Array)
#3 /var/www/pgp.key-server.io/vendor/guzzlehttp/ringphp/src/Future/CompletedFutureValue.php(55): React\Promise\FulfilledPromise->then(Object(Closure), NULL, NULL)
#4 /var/www/pgp.key-server.io/vendor/guzzlehttp/guzzle/src/Message/FutureResponse.php(43): GuzzleHttp\Ring\Future\CompletedFutureValue->then(Object(Closure), NULL, NULL)
#5 /var/www/pgp.key-server.io/vendor/guzzlehttp/guzzle/src/RequestFsm.php(134): GuzzleHttp\Message\FutureResponse::proxy(Object(GuzzleHttp\Ring\Future\CompletedFutureArray), Object(Closure))
#6 /var/www/pgp.key-server.io/vendor/guzzlehttp/guzzle/src/Client.php(165): GuzzleHttp\RequestFsm->__invoke(Object(GuzzleHttp\Transaction))
#7 /var/www/pgp.key-server.io/vendor/jenssegers/proxy/src/Adapter/Guzzle/GuzzleAdapter.php(54): GuzzleHttp\Client->send(Object(GuzzleHttp\Message\Request))
#8 /var/www/pgp.key-server.io/vendor/jenssegers/proxy/src/Proxy.php(80): Proxy\Adapter\Guzzle\GuzzleAdapter->send(Object(Symfony\Component\HttpFoundation\Request), 'http://127.0.0....')
#9 /var/www/pgp.key-server.io/src/ctubio/HKPProxy/Keyserver/Router.php(46): Proxy\Proxy->to('http://127.0.0....')
#10 /var/www/pgp.key-server.io/src/ctubio/HKPProxy/Keyserver/Router.php(14): ctubio\HKPProxy\Keyserver\Router::getHKPResponse('/pks/lookup?sea...')
#11 /var/www/pgp.key-server.io/src/ctubio/HKPProxy/Keyserver.php(19): ctubio\HKPProxy\Keyserver\Router::getResponse()
#12 /var/www/pgp.key-server.io/pub/php-proxy-keyserver.php(14): ctubio\HKPProxy\Keyserver::getResponse()
#13 {main}

Various other key servers say "No results found" to any key I request.

• What is going on? Is it a temporary problem? But why would all the key servers be offline at the same time?
• Does it have anything to do with access to every key server being blocked by my connection (Then why can I reach the web interfaces that will then fail?)?
• Is it that I am searching for invalid strings? I do not think so; I searched for a number of different ones, the one in the example here (F434104235DA97EB) should be the one that signed this package on github...
• Is it that I am not connecting to the correct key servers?