Apparently, org.onehippo.cms7.essentials.dashboard.utils.DocumentTemplateUtils#addContentMixin() tries to add a mixin to documents of the specified type, which breaks when the versionable document variant is checked in. By default, demo content installed by Essentials isn't checked in, but once you changed and published a document, it is checked in, and then, adding a taxonomy to this document type fails:
ERROR http-nio-8080-exec-1 [TaxonomyResource.addTaxonomyToDocument:249] Error adding taxonomy to a document
javax.jcr.version.VersionException: Unable to perform operation. Node is checked-in.
Above function should ensure that the nodes are checked out before adding the mixin.