I see the following error in RM.log while trying to create folder using OSLC RM API in RDNG v 6.0.1
ERROR fronting.server.services.folders.FolderRestService  - CRRRS1007E  A problem occurred in the Requirements Management (RM) server: Entry [9811367043c9bf12]
java.lang.StringIndexOutOfBoundsException: String index out of range: -1

RM URL: https://localhost:10443/rm/folders/_wW1KUFceEeeDm5R8Q4UhUw
POST Message :
<rdf:RDF
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns%23"
xmlns:dcterms="http://purl.org/dc/terms/"
xmlns:oslc="http://open-services.net/ns/core%23"
xmlns:nav="http://jazz.net/ns/rm/navigation%23">
<nav:folder rdf:about="">
<dcterms:title>Sample Folder</dcterms:title>
<nav:parent rdf:resource="https://localhost:10443/rm/folders/_wW1KUFceEeeDm5R8Q4UhUw"/></nav:folder>
</rdf:RDF>

Do you see any issue with the message am trying to send or with REST URL?. 
Thanks in advance!
Mallana