I am having the same general problem. However, it also seems to be spreading. While it began around 9/30 with just Protonmail’s login page, I now get this error for roughly half of all pages I try to load. On some, thankfully, Brave at least gives the option, under “Advanced”, to proceed to the page despite it being “unsafe”. But some, like Protonmail (and most frustratingly, Wikipedia), it tells me the problem has to do with “HSTS” instead:
You cannot visit en.wikipedia.org right now because the website uses HSTS. Network errors and attacks are usually temporary, so this page will probably work later.
…and then there is no option to proceed anyway.
As some others here have noticed, the problem does not actually seem to be localized to Brave. I have tried Safari and Ungoogled Chromium so far, and both have the same issue on generally the same sites as Brave.
Even odder, in the last day or two this problem has also appeared on my iPhone: I can no longer get Wikipedia and instead am told:
Safari cannot open the page because it could not establish a secure connection to the server.
So if this is happening to so many sites on so many browsers and BOTH my laptop and iphone, it seems like this must be some deep systemic bug that has nothing to do with anything we’ve done.
I have no idea what the “Let’s Encrypt” or “DST Root CA X3” certificate is, but I hope it gets fixed mighty soon!