Changes included in 2.1.1-2.1.4 broke API compatibility with Bitbucket Server 4.0.0-4.5.3 by introducing a reference a class that wasn't added to the API until 4.6.0. 2.1.5 now detects whether that class is present and uses it when it is while falling back to an older API method if it's not.
2.1.5 has been explicitly tested with Bitbucket Server 4.0.8, 4.14.12 and 5.0.10 (which is the newest version this app will ever support, since Bitbucket Server 5.1.0 bundles its functionality as a built-in feature).