That's a very very old PAN-OS. You can try debugging the GUI by going into debug mode and starting a capture the moment you try to download:Open 2 tabs1 on the exportOther on https://<firewall>/debugEnable debug, proceed to download, wait till failed, disable debug, refesh, check the logIf nothing pops up: upgrade to a contemporary version of PAN-OS
Are you able to export any logs or just not report of Global VPN Users?
https://knowledgebase.paloaltonetworks.com/KCSArticleDetail?id=kA10g000000Clj3CAC
Are you on a recent PAN-OS? All the devices I've tried this on allow me to download the report
did you check if your browser (or an AV software) might be blocking the download? have you tried a different browser/different computer?