Verified fixed in 5.5.2.1 - 5.5.2.1.20160114044944_395c086. Both single-host and multi-host editing, accessed through a provider relationships page, works now. Provider used for testing was vCenter 5.5.
Since the problem described in this bug report should be resolved in a recent advisory, it has been closed with a resolution of ERRATA. For information on the advisory, and where to find the updated files, follow the link below. If the solution does not work for you, open a new bug report. https://access.redhat.com/errata/RHBA-2016:0159