@stephenw10 Thanks for your reply, but I am not sure if this was the case, as I can see in the log that igb0 - igb3 is still same (original) mac address.
I also switched the two Quad NICs in the PCI-E ports. After switching the WebGui and WAN were available again and I was also able to enable to new interfaces. Everything now works.
Still do not understand why just adding new NIC in empty PCI-E didn't work immediately. Adding a new NIC shouldn't require any adjustments or analysis (imo).