[BitBucket user: Sébastien Jodogne]
[BitBucket date: 2016-11-02.09:39:36]

This is an issue with the JavaScript code of Orthanc Explorer. As a temporary workaround, you'll have to refresh your Web browser (i.e. hit F5 key) to reinitialize the state of Orthanc Explorer.

Reference: https://groups.google.com/d/msg/orthanc-users/z9EpeIeyW4k/plepXuSEDQAJ