To narrow down the problem, it’s only happening when I use Safari browser version 18.2 (20620.1.16.11.8). It works fine when I use Firefox, Edge or Chrome browsers.
I have checked Safari’s settings and I don’t see anything unusual.
I have just installed the macOS version Sequoia 15.4 on my MacPro and, surprisingly, PopClip has started working again in Safari when I am on a website where a PDF file is presented and I select text.