Jan,
A few things could cause this - were you able to upgrade before from your current setup? Are you behind a firewall and blocking any ports?
You could try to enable Developer Tools > Network on your browser and then that should show you what exactly your browser is trying to access that it's not able. It could be that your network is blocking the port, or access to that server, or that there's an SSL issue.
Let us know what you find out!
Otherwise, you can try to click the link you sent and manually download the file and import it directly into Jira.
Kind Regards,
Shannon
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