What if you complete remove this account, then reconfigure it in Outlook with a new Outlook profile?
If this happens on all the clients, then most likely it's an issue on the server side, from my opinion.
Thanks, not sure if it was this way on the iPhone, the ISP did some kind of remote session and recreated the profile which then recreated the other folders which were still on the server however the Inbox was wiped and needs to be restored from the backup.