Windows 10 update cannot connect (behind a firewall)

Why cannot Windows 10 Update connect to its update servers even though there is a working internet connectivity?

About my setup

• running Windows 10 on a PC behind a dedicated firewall (pfsense 2.2.5)


• there is internet connectivity, port 80 / 443 works fine for anything else


• connecting the Windows 10 PC directly to my IPS's modem allows to update Windows 10 => suggests that the firewall is causing the problem

Already checked

Still, Windows 10 (version 1511) cannot update itself when I run it behind my firewall.

Any other suggestions/ideas?

Answer

Answering my own question.

I found the Windows Update Troubleshooter for Win10 from MS.

While running it, it in fact found and fixed a problem. After that, my Windows 10 can update itself now (great).

So, even though everything suggested a problem with my pfSense firewall it was some problem with the Windows Update itself.