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Sorry to hear you’re facing problems 
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In order to help you as quickly as possible, before clicking Create Topic please provide as much of the below as you can. Feel free to use a pastebin service for logs, otherwise either indent short log examples with four spaces:
example
Or for longer, use three backticks above and below the code snippet:
longer
example
here
Some or all of the below information will be requested if it isn’t supplied; for fastest response please provide as much as you can 
Nextcloud version ( 15.0.5):
Operating system and version (eg, Ubuntu 18.04):
Webhost: Caddy
PHP version ( 7.2):
The issue you are facing:
Download of bigger files fail. Also speed doesn’t seem to be right. The server is connected with 400 Mbit/s and I am with 200 Mbit/s wich I usually get. I see download speeds as high as 68 Mbit/s. At some point the connection breaks and I don’t know why.
Is this the first time you’ve seen this error? (Y/N):N
Steps to replicate it:
- Setup Nextcloud with Caddy as Server
The output of your Nextcloud log in Admin > Logging:
PASTE HERE
The output of your config.php file in /etc/php/7.2/fpm/php.ini (make sure you remove any identifiable information!):
;;;;;;;;;;;;;;;;
; File Uploads ;
;;;;;;;;;;;;;;;;
; Whether to allow HTTP file uploads.
; http://php.net/file-uploads
file_uploads = On
; Temporary directory for HTTP uploaded files (will use system default if not
; specified).
; http://php.net/upload-tmp-dir
upload_tmp_dir = /mnt/data/nextcloud
; Maximum allowed size for uploaded files.
; http://php.net/upload-max-filesize
upload_max_filesize = 16G
; Maximum number of files that can be uploaded via a single request
max_file_uploads = 50
opcache.enable=1
opcache.enable_cli=1
opcache.interned_strings_buffer=8
opcache.max_accelerated_files=10000
opcache.memory_consumption=128
opcache.save_comments=1
opcache.revalidate_freq=1
php_value upload_max_filesize 16G
php_value post_max_size 16G
php_value max_input_time 3600
php_value max_execution_time 3600
/var/www/nextcloud
PASTE HERE
The output of yourCaddy log:
None, no errors.
Caddy config:
cloud.xy.at {
root /var/www/nextcloud
fastcgi / /run/php/php7.2-fpm.sock php {
fastcgi / 127.0.0.1:9000 php {
env PATH /bin
}
rewrite {
r ^/index.php/.*$
to /index.php?{query}
}
# client support (e.g. os x calendar / contacts)
redir /.well-known/carddav /remote.php/carddav 301
redir /.well-known/caldav /remote.php/caldav 301
# remove trailing / as it causes errors with php-fpm
rewrite {
r ^/remote.php/(webdav|caldav|carddav)(\/?)$
to /remote.php/{1}
}
rewrite {
r ^/remote.php/(webdav|caldav|carddav)/(.+)(\/?)$
to /remote.php/{1}/{2}
}
.htacces / data / config / ... shouldn't be accessible from outside
rewrite {
r ^/(?:\.htaccess|data|config|db_structure\.xml|README)
status 403
}
header / {
Strict-Transport-Security “max-age=31536000; includeSubdomains; preload;”
# Enable cross-site filter (XSS) and tell browser to block detected attacks
X-XSS-Protection “1; mode=block”
# Prevent some browsers from MIME-sniffing a response away from the declared Content-Type
X-Content-Type-Options “nosniff”
# Disallow the site to be rendered within a frame (clickjacking protection)
X-Frame-Options “DENY”
Referrer_Policy “no-referrer”
X-Frame-Options “sameorigin”
}