[horde] Outlook sending constantly same meeting invitation

[Volker Then]

Hi there,

I have the problem that Outlook (desktop and mobile version, connected  
via ActiveSync) sends the same meeting invitation dozens of times.  
Someone of my clients reports that as soon as the required participant  
accepts the invitation this behaviour stops. Same behaviour, when a  
horde user accepts an invitation. The response message stays in the  
outbox and is sent once again every minute.

We are only syncing calendar and contact data, email sync is off on  
that horde server.

Any known workaround?

Thanks,

Volker