Nets, the operator of Norwegian BankID, has sent a post mortem, which includes this description of what went wrong:
As preparation of the handover of BankID to DXC, a database synchronization on
pre-prod is run between Nets and DXC. The synchronization caused saturation in
network and Firewall 3 which in turn interfered with BankID traffic in production.
As a consequence of this problem they have taken the following action:
It has been agreed with Database and BankID team that these synchronizations
will only be run between 00:00 and 07:00 to limit impact on services.