USPTO has published a content-free page about the massive system outage that (among other things) has broken Private PAIR and EFS-Web:
At approximately 4:30 a.m. ET on August 15, 2018, PALM experienced an issue during maintenance that impacted its ability to function properly.
Yes, that tells us a lot about what happened (not!).
It is sort of true that the EFS-Web Contingency Server is sort of working. I successfully e-filed a design application on Friday using that server — or at least I think I succeeded. Of course I will not really know whether I succeeded until USPTO gets Private PAIR working again.
There are anecdotal reports that the EFS-Web Contingency Server does not work for entry into the US national phase.
See two webinars about workarounds, for which recordings are available.
The chief takeaway from the content-free page is that USPTO commits to posting updates every day at 10AM and 4PM until things are back to normal.