The regular webgui service call should work equally well. Try removing the question mark at the end of the URL, or simply start transaction WEBGUI which launches SAP GUI for HTML and check the proper URL that way.
The main reason for using the personas service originates from the time when it was necessary to pre-configure webgui for Personas with certain parameters for proper operation. This is mostly needed for older basis releases still. On a more modern basis system, this is not really needed anymore and launching webgui is just fine. In fact, in a public cloud instance, there is no separate personas service available and the only way to access Personas is via using the webgui service.
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