Changing licenses.json in ckan.ini throws Internal Server Error

[HMinD-V3]

We're new to CKAN, but got a 2.10 installation up and running. When changing the licenses_group_url in ckan.ini (either to local file or https request), first the respective licenses are selectable correctly in the dataset edit form, but when saving the dataset, we get an Internal Server Error. Currently, only the built-in license set works, or can be successfully requested from here. This one, however, causes the named error, although it's valid JSON and has the same structure. What's wrong with it?

This leads to some related questions:

1. What is the usual folder to store the JSON file? We tried e.g. file:///usr/lib/ckan/default/src/ckan, but no success.
2. Is there a way to get more elaborate error messages than just "Internal server error"?
3. ckan.ini is abviously working, but is it at the right place in /etc/ckan/default?

Sorry for the extensive questions, but we hope that any CKAN pro will answer them at a glance.