[SOLVED] tls certificate of https://download.proxmox.com/

1. we know, that we should use http://download.proxmox.com but our it department does no longer allow http so we have to use https.

Just use the enterprise repo or re-check your requirements. The packages are validated on install, so no technical need for https.

2. we have an official subscription for our 3 proxmox production servers.

Great!

3. we have a proxmox test server without a subscription and this is the questionable machine
If need https, just go for a quite affordable community subscription to get your IT department happy.
anyway, the tls certificate of https://download.proxmox.com/ does not match the hostname.
there is no tls certificate for download, if you really need https (again, no security benefit), go for a subscription. but yes, do avoid such discussions, we think of improvements in future (not that easy to implement due to the design of our CDN).