Hi Guus,
Seems like you have an issue with default virtual hostname , this should be the FQDN of the server (ie yz1web.ijzereef.nl)
You may check the to the default server assigned domainname with running httpd -S.
Note: we implemented the redirect to /SOGo in the virtual host configuration of nethserver-sogo. If you (still) have custom templates or other customization to accomplish this, it is better to remove them.
(do not forget to run signal-event nethserver-sogo-update after your edits 
)
If your system is up to date the SOGo panel made by @stephdl can be found in the service manager;
where (if desirable ) a hostname for SOGo can be set.