Today we finally solved the problems we had with git.
The problems were that when we merged a clone of UnitySteer into our repository, pushed it and then pulled it on another computer it arised several compile errors in the scripts from UnitySteer. After several tries we managed to solve this problem.
We had a problem with the linking of the UnitySteer scripts in our agent prefab when pulling from git. The script components were still attatched to the agent but they were empty. This was due to some changes in the meta files in the UnitySteer package. When we renewed the UnitySteer package this problem was solved.