Here's the final update regarding the OS upgrades for now:

Valdivia and Johannesburg are back online. Frankfurt (01) is back online as well with an updated IP address.

Replacing the Fremont (01) resolver by a new resolver in San Jose is still pending.

Düsseldorf (01) will stay as-is for now. The provider doesn't offer me an option to install a newer OS at the moment. For good measure, please switch to Düsseldorf (02).