Hi Karsten,
When I accessed the public website with IE, there was an alert message in the debugging log “you have not whitelisted this domain for your Twitter widget. Go to https://twitter.com/settings/widgets/285381470439219200/edit and add 'portal.kopwerk.nl' to the 'Domains' list for this timeline.” , so I suggest you follow up this information to narrow down the issue first.
Thanks,
Eben Liu
