This is a confusing issue because it's NOT happening on other machines within the same network (same ISP, etc). I go as even as far as testing on a virtual on the same machine that is having the issue within the host operating system. I'm not getting this error within the Virtual, but getting it on the host OP that is the same machine. I was told to un-install/install and I've done this several times, still not working.
Assuming you are talking about a client on the Internet connecting to your public GP Portal.
Did you have a look at the PanGP Service and PanGP Agent Logs? Those are very verbose and shall give you the answer of your question. Open the GlobalProtect client --> Settings --> Troubleshooting tab :
Problems usually are around the DNS name / FQDN / URL that is not reachable and certificates not valid.
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