current Virtualization Support Matrix confirms your findings:
* Data Protector GRE Web Plug-in is supported only on vCenter Server versions upto 5.5.0 and not higher versions.
**Data Protector Advanced GRE Web Plug-in is supported only on Vcenter Server versions 5.5.1 or higher versions.
Looks to me you have to upgrade to 5.5.1 and use the new Advanced GUI or go back to 5.5.0u1 and use the old one.