Okay solved the problem myself. It may be useful for someone else.
I am a bit paranoid so my server have a strict firewall blocking anything that has not been white listed. Even on output.
Horde need to synchronize the timezone using caldav. There for it tries to get timezone infos using a ftp:
$conf[‘timezone’][‘location’] =‘ftp://ftp.iana.org/tz/tzdata-latest.tar.gz’;
It was blocked by my firewall and therefore blocking sync.
thanks for the app
