1. Is the protocol(http, https etc.) necessary?
Yes it is, as we are validating on the base URL for example: https://servername.domain.de/
2. What if the we also accesses/can access the Qlik Sense server via IP address?
Then the URL would be part of IP address e.g. http://192.168.1.XXX/
3. Shall we also enter the hostname in any case? Even if we have e.g. "qlik.company.com" but we also access it also via e.g. "vmname.company.local".
That forms part of any URL structure. So in the above case, it will be http://vmname.company.local or https://vmname.company.local
4. Regarding hostnames: Will the hostname ("vmname") itself suffice, or does it have to be the FQDN ("vmname.company.local")?
Whatever URL you use to access Qlik is the URL you use for licensing.
5. In case of a Qlik Sense cluster: Do we also need to give the URLs (hostnames) of all the nodes, right?
Yes, you are able to add multiple URLs. When downloading the extension, you can download it with several URLs attached to have a combined extension with multiple URLs