I had to reboot (normal after the update)
So Gardian was in errror
Need to run pakfire again : seems OK
New reboot
Saw that Guardian service was not started
Manual start of Guardian : OK
If so, it might be that the upgrade to Core Update 171 caused, for example, the termination of VPN connections. In such a case (presumed that the upstream proxy is behind a VPN), immediate attempts of Pakfire to update all the add-ons as well will fail.
As @steven pointed out, updating the add-ons should work fine at a later point in time, where the necessary preconditions for fetching package lists are met again.