Issues with connecting to MS activation service and AMD update systems

I set up a brand new PC and installed Windows 11 Pro 25H2 on it. When trying to connect to the Microsoft Activation service I run into issue. Same happened when trying to install latest driver updates from AMD.
Both tasks could be completed via hot-spotting to a MiFi device, so likely IPFire or my configuration of it was causing the issue. After some investigation it seems that at least for the Microsoft side there is an issue with the certificate chain:

openssl s_client -connect ``activation.sls.microsoft.com:443`` -servername ``activation.sls.microsoft.com`` CONNECTED(00000003) depth=0 C = US, ST = WA, L = Redmond, O = Microsoft, OU = WSE, CN = ``validation-v2.sls.microsoft.com``, CN = ``activation-v2.sls.microsoft.com``, CN = ``validation.sls.microsoft.com``, CN = ``activation.sls.microsoft.com`` verify error:num=20:unable to get local issuer certificate verify return:1 depth=0 C = US, ST = WA, L = Redmond, O = Microsoft, OU = WSE, CN = ``validation-v2.sls.microsoft.com``, CN = ``activation-v2.sls.microsoft.com``, CN = ``validation.sls.microsoft.com``, CN = ``activation.sls.microsoft.com`` verify error:num=21:unable to verify the first certificate verify return:1 depth=0 C = US, ST = WA, L = Redmond, O = Microsoft, OU = WSE, CN = ``validation-v2.sls.microsoft.com``, CN = ``activation-v2.sls.microsoft.com``, CN = ``validation.sls.microsoft.com``, CN = ``activation.sls.microsoft.com

Anybody an idea how to solve this?

Best regards,

Oliver