Error: HTTP_Request2 needs an absolute HTTP(S) request URL, '/rest/registry' giv

Hi,

The “Test Settings” Button returns without any error message beside this PHP-Error itself:
Error: HTTP_Request2 needs an absolute HTTP(S) request URL, ‘/rest/registry’ given

If I click the G3-Button a white page opens with no content.
What could be the problem here?

Thanks and kind regards

maus

https://ww.wp.xz.cn/plugins/heiv-gallery-3/