When a Hg revision is transplanted using the Transplant extension (not Graft), the changeset hash of the original revision gets included in binary form in the Extras field of the transplanted revision. Typically this binary hash cannot be assumed to consist of only valid unicode characters.
Testcase: Create a repository on scm-manager, and push the contents of the public hgsubversion repository into it. Then, execute this REST api query: http://<location of scm-manager instance>/api/rest/repositories/<ID of hgsubversion repository>/changeset/8687c5aa4f35
Result is HTTP/500 due to Unicode decoding error. The "extras" field of this revision looks like this: "branch=default,transplant_source=7‘Fl>ç„ž4†äĺV~!ëúŐ" (binary data after transplant_source=)
Also, if you select the affected repository in the SCM Manager web UI, and click "Commits", then go to the page with the affected revision, an error message is raised, for the same reasons.