Map service cannot be started after activating the Mapservice Connector

Problem

As soon as a certain new layer is included in the map service, the Mapservice Connector of this map service fails to load. The following error message appears:

An item with the same key has already been added

This is a bug in older versions when multiple data sources are used.

Workaround

Use features from the same data source.

Solution

Use at least GEONIS server 2017.2.