Hi,
Moodle does not accept internationalized domain names so I can't set a link to http://www.kölnpercussion.de ... this is an existing URL and modern browsers allow to open these URLs without conversation.
Ralf

Replication steps

1. Log in as a teacher/admin
2. Create a new URL resource
3. Use the URL http://www.kölnpercussion.de
4. Click save

Expected outcome: The URL resource should be created

Actual outcome: The External URL field is reported to be invalid