I have encountered an issue with the calendar sync. If a series appointment is sent and then one appointment out of the series is deleted, although it removes that specific appointment from outlook it does not remove it from sogo online nor phone calendar.

Steps to recreate:
1. Created an appointment series in external email account (in my case I utilized gmail)
2. Accept the series appointment in outlook (double checked that the appointments were added on sogo online and calendar)
3. In gmail deleted 1 of the appointments out of the series
4. Accepted deletion in outlook
5. Check outlook calendar and confirm specific appointment has been removed
6. Check sogo online and phone calendar and in both that specific appointment still exist

My Config
# Settings for SOGo Groupware

# SOGo
location ~ ^/sogo { rewrite ^ https://$host/SOGo; }
location ~ ^/SOGO { rewrite ^ https://$host/SOGo; }

# For Mac OS X and iOS devices.
rewrite ^/.well-known/caldav    /SOGo/dav permanent;
rewrite ^/.well-known/carddav   /SOGo/dav permanent;
rewrite ^/principals/           /SOGo/dav permanent;

location ^~ /SOGo {
    proxy_pass http://127.0.0.1:20000;
    #proxy_redirect http://127.0.0.1:20000/SOGo/ /SOGo;

    # forward user's IP address
    proxy_set_header X-Real-IP $remote_addr;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header Host $host;
    proxy_set_header x-webobjects-server-protocol HTTP/1.0;
    proxy_set_header x-webobjects-remote-host 127.0.0.1;
    proxy_set_header x-webobjects-server-name $server_name;
    proxy_set_header x-webobjects-server-url $scheme://$host;
}

location ^~ /Microsoft-Server-ActiveSync {
    proxy_pass http://127.0.0.1:20000/SOGo/Microsoft-S … ctiveSync;
    proxy_redirect http://127.0.0.1:20000/Microsoft-Server-ActiveSync /;

    proxy_connect_timeout 360;
    proxy_send_timeout 360;
    proxy_read_timeout 3540;
}

location ^~ /SOGo/Microsoft-Server-ActiveSync {
    proxy_pass http://127.0.0.1:20000/SOGo/Microsoft-S … ctiveSync;
    proxy_redirect http://127.0.0.1:20000/SOGo/Microsoft-Server-ActiveSync /;

    proxy_connect_timeout 3540;
    proxy_send_timeout 3540;
    proxy_read_timeout 3540;
}

location /SOGo.woa/WebServerResources/ {
    alias /usr/lib/GNUstep/SOGo/WebServerResources/;
}
location /SOGo/WebServerResources/ {
    alias /usr/lib/GNUstep/SOGo/WebServerResources/;
}
location ^/SOGo/so/ControlPanel/Products/([^/]*)/Resources/(.*)$ {
    alias /usr/lib/GNUstep/SOGo/$1.SOGo/Resources/$2;
}