A few things to check when configured to use HOTADD transport mode..
1. HotAdd transport is supported only on a virtual machine backup proxy
(refer http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1035096)
2. no anti-virus should be running on backup proxy host during the backup
3. automount should also be disabled on windows backup proxy host
(refer http://pubs.vmware.com/vsphere-60/index.jsp?topic=%2Fcom.vmware.vddk.pg.doc%2FvddkBkupVadp.9.6.html&resultof=%22vss%22%20)