From the error text it looks like your Tableau Server host
couldn't recognise the DNS name ows.terrestris.de as a valid one.
It may be network connectivity issues, or your TS host may be configured
to have a restricted access to external hosts/nets.
Please ask your network admin about it.
Also, you may want to check if the issue is linked specifically to your TS
by opening your workbook in question from the Tableau Desktop copy
running on the same host where your Tableau Server is running.
Please ask your TS admin for help with installing TD on the host and testing.