nginx+http2: Upstream prematurely closed connection, PROTOCOL_ERROR

On my server I have nginx running using the default docker nginx image (version 1.13.8) as a reverse proxy for Lychee (using wonderfall/lychee), a photo gallery software written in PHP. As far as I can tell, the Lychee image itself also runs nginx with php7.1-fpm.

The gallery works fine, only when trying to download an image or an album, I get a strange error in the browser. Chromium says “Failed - Network error”, and other browsers give similar error messages. This error comes up when the download has finished, and when using a browser that keeps failed downloads, it can be seen that the downloaded file is actually complete.

The error only comes up when accessing Lychee through nginx and when using HTTP/2. The nginx log reports:

2018/01/15 15:37:34 [error] 8#8: *2960 upstream prematurely closed connection while sending to client, client: 192.168.255.1, server: example.com, request: "GET /2018/php/index.php?function=Photo::getArchive&photoID=123&password= HTTP/2.0", upstream: "http://10.93.1.23:8888/php/index.php?function=Photo::getArchive&photoID=123&password=", host: "example.com"

The Lychee log reports nothing.

When trying to download a file using curl -v, this is the output:

*   Trying 1.2.3.4...
* TCP_NODELAY set
* Connected to example.com (1.2.3.4) port 443 (#0)
* ALPN, offering h2
* ALPN, offering http/1.1
* successfully set certificate verify locations:
*   CAfile: /etc/ssl/certs/ca-certificates.crt
  CApath: none
} [5 bytes data]
* TLSv1.2 (OUT), TLS handshake, Client hello (1):
} [226 bytes data]
* TLSv1.2 (IN), TLS handshake, Server hello (2):
{ [106 bytes data]
* TLSv1.2 (IN), TLS handshake, Certificate (11):
{ [2754 bytes data]
* TLSv1.2 (IN), TLS handshake, Server key exchange (12):
{ [556 bytes data]
* TLSv1.2 (IN), TLS handshake, Server finished (14):
{ [4 bytes data]
* TLSv1.2 (OUT), TLS handshake, Client key exchange (16):
} [37 bytes data]
* TLSv1.2 (OUT), TLS change cipher, Client hello (1):
} [1 bytes data]
* TLSv1.2 (OUT), TLS handshake, Finished (20):
} [16 bytes data]
* TLSv1.2 (IN), TLS handshake, Finished (20):
{ [16 bytes data]
* SSL connection using TLSv1.2 / ECDHE-RSA-CHACHA20-POLY1305
* ALPN, server accepted to use h2
* Server certificate:
*  subject: CN=example.com
*  start date: Dec  4 10:54:54 2017 GMT
*  expire date: Mar  4 10:54:54 2018 GMT
*  subjectAltName: host "example.com" matched cert's "example.com"
*  issuer: C=US; O=Let's Encrypt; CN=Let's Encrypt Authority X3
*  SSL certificate verify ok.
* Using HTTP2, server supports multi-use
* Connection state changed (HTTP/2 confirmed)
* Copying HTTP/2 data in stream buffer to connection buffer after upgrade: len=0
} [5 bytes data]
* Using Stream ID: 1 (easy handle 0x55e8df551160)
} [5 bytes data]
> GET /2018/php/index.php?function=Photo::getArchive&photoID=123&password= HTTP/2
> Host: example.com
> User-Agent: curl/7.57.0
> Accept: */*
> 
{ [5 bytes data]
* Connection state changed (MAX_CONCURRENT_STREAMS updated)!
} [5 bytes data]
< HTTP/2 200 
< server: nginx/1.13.8
< date: Mon, 15 Jan 2018 15:37:34 GMT
< content-type: application/octet-stream
< content-length: 530149
< x-powered-by: PHP/7.1.2
< set-cookie: PHPSESSID=123; path=/2018/
< expires: Thu, 19 Nov 1981 08:52:00 GMT
< cache-control: no-store, no-cache, must-revalidate
< pragma: no-cache
< content-disposition: attachment; filename="IMG_4872.jpg"
< strict-transport-security: max-age=31536000
< 
{ [3691 bytes data]
* HTTP/2 stream 1 was not closed cleanly: PROTOCOL_ERROR (err 1)
* Closing connection 0
} [5 bytes data]
* TLSv1.2 (OUT), TLS alert, Client hello (1):
} [2 bytes data]

When downloading with HTTP/1.1, everything works fine:

*   Trying 1.2.3.4...
* TCP_NODELAY set
* Connected to example.com (1.2.3.4) port 443 (#0)
* ALPN, offering http/1.1
* successfully set certificate verify locations:
*   CAfile: /etc/ssl/certs/ca-certificates.crt
  CApath: none
} [5 bytes data]
* TLSv1.2 (OUT), TLS handshake, Client hello (1):
} [223 bytes data]
* TLSv1.2 (IN), TLS handshake, Server hello (2):
{ [112 bytes data]
* TLSv1.2 (IN), TLS handshake, Certificate (11):
{ [2754 bytes data]
* TLSv1.2 (IN), TLS handshake, Server key exchange (12):
{ [556 bytes data]
* TLSv1.2 (IN), TLS handshake, Server finished (14):
{ [4 bytes data]
* TLSv1.2 (OUT), TLS handshake, Client key exchange (16):
} [37 bytes data]
* TLSv1.2 (OUT), TLS change cipher, Client hello (1):
} [1 bytes data]
* TLSv1.2 (OUT), TLS handshake, Finished (20):
} [16 bytes data]
* TLSv1.2 (IN), TLS handshake, Finished (20):
{ [16 bytes data]
* SSL connection using TLSv1.2 / ECDHE-RSA-CHACHA20-POLY1305
* ALPN, server accepted to use http/1.1
* Server certificate:
*  subject: CN=example.com
*  start date: Dec  4 10:54:54 2017 GMT
*  expire date: Mar  4 10:54:54 2018 GMT
*  subjectAltName: host "example.com" matched cert's "example.com"
*  issuer: C=US; O=Let's Encrypt; CN=Let's Encrypt Authority X3
*  SSL certificate verify ok.
} [5 bytes data]
> GET /2018/php/index.php?function=Photo::getArchive&photoID=1234&password= HTTP/1.1
> Host: example.com
> User-Agent: curl/7.57.0
> Accept: */*
> 
{ [5 bytes data]
< HTTP/1.1 200 OK
< Server: nginx/1.13.8
< Date: Mon, 15 Jan 2018 15:43:36 GMT
< Content-Type: application/octet-stream
< Content-Length: 530149
< Connection: keep-alive
< X-Powered-By: PHP/7.1.2
< Set-Cookie: PHPSESSID=1234; path=/2018/
< Expires: Thu, 19 Nov 1981 08:52:00 GMT
< Cache-Control: no-store, no-cache, must-revalidate
< Pragma: no-cache
< Content-Disposition: attachment; filename="IMG_4872.jpg"
< Strict-Transport-Security: max-age=31536000
< 
{ [3691 bytes data]
* Connection #0 to host example.com left intact

Finally, here is the output when accessing Lychee directly:

*   Trying 10.93.1.23...
* TCP_NODELAY set
* Connected to 10.93.1.23 (10.93.1.23) port 8888 (#0)
> GET /php/index.php?function=Photo::getArchive&photoID=1234&password= HTTP/1.1
> Host: 10.93.1.23:8888
> User-Agent: curl/7.56.1
> Accept: */*
> 
< HTTP/1.1 200 OK
< Server: nginx
< Date: Mon, 15 Jan 2018 15:46:06 GMT
< Content-Type: application/octet-stream
< Content-Length: 530149
< Connection: keep-alive
< X-Powered-By: PHP/7.1.2
< Set-Cookie: PHPSESSID=1234; path=/
< Expires: Thu, 19 Nov 1981 08:52:00 GMT
< Cache-Control: no-store, no-cache, must-revalidate
< Pragma: no-cache
< Content-Disposition: attachment; filename="IMG_4872.jpg"
< 
{ [14070 bytes data]
* Connection #0 to host 10.93.1.23 left intact

I don't understand why nginx says that Lychee prematurely closed the connection, when actually the whole file has been transfered (the Content-Length matches also), and running curl by hand also doesn't show any errors. Does anyone have an idea?