Do you use Apache? Have you set the x-webobjects-server-port and x-webobjects-server-url headers in your proxy?

Yes, we are using Apache and headers are set. Content of /etc/httpd/conf.d/SOGo.conf here:

Alias /SOGo.woa/WebServerResources/ \
/usr/lib64/GNUstep/SOGo/WebServerResources/
Alias /SOGo/WebServerResources/ \
/usr/lib64/GNUstep/SOGo/WebServerResources/

<Directory /usr/lib64/GNUstep/SOGo/>
AllowOverride None
<IfVersion < 2.4>
Order deny,allow
Allow from all
</IfVersion>
<IfVersion >= 2.4>
Require all granted
</IfVersion>

<IfModule expires_module>
  ExpiresActive On
  ExpiresDefault "access plus 1 year"
</IfModule>

</Directory>

ProxyRequests Off
SetEnv proxy-nokeepalive 1
ProxyPreserveHost On

ProxyPass /SOGo http://127.0.0.1:20000/SOGo retry=0

<Proxy http://127.0.0.1:20000/SOGo>
RequestHeader set "x-webobjects-server-port" "443"
RequestHeader set "x-webobjects-server-name" "%{HTTP_HOST}e" env=HTTP_HOST
RequestHeader set "x-webobjects-server-url" "https://%{HTTP_HOST}e&quot; env=HTTP_HOST
RequestHeader unset "x-webobjects-remote-user"
RequestHeader set "x-webobjects-server-protocol" "HTTP/1.0"
AddDefaultCharset UTF-8

Order allow,deny
Allow from all
</Proxy>

<IfModule rewrite_module>
RewriteEngine On
RewriteRule ^/.well-known/caldav/?$ /SOGo/dav [R=301]
RewriteRule ^/.well-known/carddav/?$ /SOGo/dav [R=301]
</IfModule>

Any suggestion/solution? Users are not able to get their attachments and it makes them a bit frustrated.