Nginx – SSL alert number 113

We're seeing some errors in the past few days similar to this one, in our nginx error logs:

/var/log/nginx/error.log.2.gz:2017/01/30 16:11:46 [crit] 13114#13114: *139338 SSL_do_handshake() failed (SSL: error:14094459:SSL routines:SSL3_READ_BYTES:tlsv1 bad certificate status response:SSL alert number 113) while SSL handshaking, client: X.X.X.X, server: 0.0.0.0:443

We're using Let's Encrypt for this certificate. We can't reproduce this problem ourselves, so far we haven't been able to get any information about what might be causing this from the client side.

RFC 6066 says that this is related to OSCP:

Clients requesting an OCSP response and receiving an OCSP response in
a "CertificateStatus" message MUST check the OCSP response and abort
the handshake if the response is not satisfactory with
bad_certificate_status_response(113) alert. This alert is always
fatal.

We have this in our nginx config:

# OCSP Stapling
# fetch OCSP records from URL in ssl_certificate and cache them
ssl_stapling on;
ssl_stapling_verify on;

The domain gets an A+ from SSL Labs, and we can't reproduce this ourselves. What could cause this error?

Edit: For the 3 times this has happened in the last few days, only one has left an entry for its IP address in the access log:

/var/log/nginx/access.log:X.X.X.X - - [01/Feb/2017:12:12:51 -0500] "GET /images/foo/bar.png HTTP/1.1" 200 6174 "-" "Mozilla/5.0 (BB10; Kbd) AppleWebKit/537.35+ (KHTML, like Gecko) Version/10.3.2.2639 Mobile Safari/537.35+"

Edit 2: This is the output of openssl s_client -connect <address>:<port> -showcerts -status:

$ openssl s_client -connect foo.bar.com:443 -showcerts -status
CONNECTED(00000003)
depth=1 /C=US/O=Let's Encrypt/CN=Let's Encrypt Authority X3
verify error:num=20:unable to get local issuer certificate
verify return:0
OCSP response: 
======================================
OCSP Response Data:
    OCSP Response Status: successful (0x0)
    Response Type: Basic OCSP Response
    Version: 1 (0x0)
    Responder Id: C = US, O = Let's Encrypt, CN = Let's Encrypt Authority X3
    Produced At: Feb  2 02:49:00 2017 GMT
    Responses:
    Certificate ID:
      Hash Algorithm: sha1
      Issuer Name Hash: 7EE66AE7729AB3FCF8A220646C16A12D6071085D
      Issuer Key Hash: A84A6A63047DDDBAE6D139B7A64565EFF3A8ECA1
      Serial Number: 0320C25EEBD8FE0BBC3678CC437E182E6D82
    Cert Status: good
    This Update: Feb  2 02:00:00 2017 GMT
    Next Update: Feb  9 02:00:00 2017 GMT

    Signature Algorithm: sha256WithRSAEncryption
        6b:10:31:84:c6:ec:32:2f:60:b2:5e:a9:a9:af:96:09:0d:53:
        7d:1d:9d:25:4e:2a:c2:46:72:51:57:ae:62:d0:6f:b8:ae:0c:
        50:d1:6f:f1:84:1f:8b:c8:fb:ed:08:8b:2f:8f:9d:d4:39:31:
        dc:6c:f5:99:27:d1:39:cb:f6:e8:c0:db:5e:99:e8:df:74:96:
        79:5a:19:ae:b7:84:bc:e2:ff:66:da:1d:dc:ad:d5:90:af:d7:
        30:83:28:65:fa:12:0e:46:5d:b4:4d:e0:a2:b8:75:3c:f9:15:
        9e:b3:12:28:34:01:0c:53:05:ee:2a:26:d4:81:fb:9c:62:9b:
        d6:43:15:ab:a1:cb:f7:ca:e5:6b:4b:7d:79:dd:72:39:93:1e:
        3f:e7:74:70:c5:de:79:27:db:79:bf:16:c8:ea:c4:a0:c7:d8:
        f1:5c:91:61:dd:4f:67:65:2f:4d:eb:76:8e:9d:ff:99:32:3d:
        41:7d:35:e9:25:5b:c1:c6:b3:30:c4:8c:9f:56:8b:86:65:4f:
        16:5f:b2:84:d3:f5:24:d9:9e:4f:b2:57:2a:e0:ee:67:01:e8:
        72:1b:ad:fd:c8:fd:a9:d5:7c:a4:bb:aa:be:96:22:83:c7:d5:
        36:82:51:27:f0:9f:00:9b:51:63:6c:39:02:29:dd:cc:7b:a9:
        62:7a:03:ee
======================================
---
Certificate chain
 0 s:/CN=foo.bar.com
   i:/C=US/O=Let's Encrypt/CN=Let's Encrypt Authority X3
-----BEGIN CERTIFICATE-----
MIIFAjCCA+qgAwIBAgISAyDCXuvY/gu8NnjMQ34YLm2CMA0GCSqGSIb3DQEBCwUA
MEoxCzAJBgNVBAYTAlVTMRYwFAYDVQQKEw1MZXQncyBFbmNyeXB0MSMwIQYDVQQD
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eS8wDQYJKoZIhvcNAQELBQADggEBAAZB906DvRm88EojJtMPR4j8JaqS9tMjIrR7
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LgrUWSq9Oy/1OK/r/W+t3Igr6uv+PXQPNPz7/Wf6h52h60JnbmnwJjQYhkoGbFKI
ZLjsDnwKMvsZqw6ya9NscebsQAwo07/DTlKuY4Gvo+vBmhVxliBjvs5TkXMpWGr+
E1qIzL+vYDborgOFwPBiDW1cpqrNCf6RdPYCbcmiq5YpHd6qD/0=
-----END CERTIFICATE-----
 1 s:/C=US/O=Let's Encrypt/CN=Let's Encrypt Authority X3
   i:/O=Digital Signature Trust Co./CN=DST Root CA X3
-----BEGIN CERTIFICATE-----
MIIEkjCCA3qgAwIBAgIQCgFBQgAAAVOFc2oLheynCDANBgkqhkiG9w0BAQsFADA/
MSQwIgYDVQQKExtEaWdpdGFsIFNpZ25hdHVyZSBUcnVzdCBDby4xFzAVBgNVBAMT
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KOqkqm57TH2H3eDJAkSnh6/DNFu0Qg==
-----END CERTIFICATE-----
---
Server certificate
subject=/CN=foo.bar.com
issuer=/C=US/O=Let's Encrypt/CN=Let's Encrypt Authority X3
---
No client certificate CA names sent
---
SSL handshake has read 4125 bytes and written 435 bytes
---
New, TLSv1/SSLv3, Cipher is DHE-RSA-AES256-SHA
Server public key is 2048 bit
Secure Renegotiation IS supported
Compression: NONE
Expansion: NONE
SSL-Session:
    Protocol  : TLSv1
    Cipher    : DHE-RSA-AES256-SHA
    Session-ID: 4F251FC1206A7455B45ABB58137F8EBFE0E23980C8C5FA2185F849AC92E99E39
    Session-ID-ctx: 
    Master-Key: 0C7B5BA714DAFA5791BA956DBC4BD642B6CABA21CB6622172B65AC3BACB063D910F38DA1D63E5A90B2C209FE442B5294
    Key-Arg   : None
    TLS session ticket lifetime hint: 300 (seconds)
    TLS session ticket:
    0000 - 6e fe 98 71 de f9 22 6f-c6 6c b2 75 fb 94 96 3b   n..q.."o.l.u...;
    0010 - 8e 35 66 14 6c c5 01 29-29 b8 fc 19 f7 dd 5a d8   .5f.l..)).....Z.
    0020 - 6f 5b 5d f9 0c 55 f5 61-af 7e a3 fa 71 f1 7e a8   o[]..U.a.~..q.~.
    0030 - 61 26 ac ab fc a8 6a b0-43 da 47 fe 73 88 85 5e   a&....j.C.G.s..^
    0040 - 05 c5 15 30 3a 24 35 dc-60 30 eb 08 1a 1a 96 73   ...0:$5.`0.....s
    0050 - 08 98 83 56 86 cf b4 c5-17 42 8c fd a3 f9 02 89   ...V.....B......
    0060 - 2d d3 75 1d 54 10 91 04-37 65 41 a2 02 7a 6d 4d   -.u.T...7eA..zmM
    0070 - db 52 b2 46 67 cb ab 32-39 5f e8 e2 3f 98 5f 1b   .R.Fg..29_..?._.
    0080 - 69 e7 91 9a cd 76 03 85-09 79 cb c0 85 96 b1 f1   i....v...y......
    0090 - c4 bc 18 31 a5 0a 46 d5-4f 22 fd 70 7e 5d 68 08   ...1..F.O".p~]h.
    00a0 - 38 5b 36 66 8c ad e9 3a-e5 51 1a aa db 77 08 7d   8[6f...:.Q...w.}

    Start Time: 1486065610
    Timeout   : 300 (sec)
    Verify return code: 0 (ok)
---
closed