I was lucky to have a couple of laptops to experiment with. On one I upgraded to Big Sur without installing GlobalProtect first. Only 5.2.3 worked for me, 5.0 and 5.1.7 did not work. It opens, and finds the firewall, but after sign in just says it cannot connect. However, on another laptop, I had GlobalProtect 5.1.7 installed and running on Catalina, then upgraded to Big Sur and then 5.1.7 was working.
For all cases, I had to go to System Preferences, Security and Privacy5., General and clicked 'Allow'. On Catalina OS, 5.1.7 would eventually pop up windows to ask permission to access certain folders to which I clicked 'Ok'. In Big Sur for the fresh install, I never got those windows to pop up, which may be the problem.
But again, 5.2.3 is working now, just had to click 'Allow' under the General tab in Security and Privacy.
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