Maybe consolidating ~60% of the web's certificates on to a single provider was a mistake.

Well good thing everyone using the provider is using an open protocol and it's stupid easy to switch

Which free CA should I use instead of lets encrypt that has same browser support?

Actalis, based in Italy offers a free tier, with ACME https://www.actalis.com/subscription

ZeroSSL from Austria also has a limited free tier. https://zerossl.com/pricing/

I mean really, if you use lets encrypt for anything that runs in a production environment, the responsible thing to do is build a fallback to switch to another provider in case LE has a bad day (or hits a brick wall and needs to say, enforce export restrictions).

Worth noting that Actalis requires you to register an account with them in order to acquire the necessary authorization token for their ACME API. This poses a privacy/anonymity issue for some users. Last I checked, Actalis' free tier didn't support SAN either.

Add.: I created an account just now to see "what's what" and also found the notice, "Activate your free 90 days certificates. At the end of the free year, the services associated with the certificates will expire." which sort of sounds like it's just a 1-year free trial.

ZeroSSL / BuyPass

Buypass no longer issues TLS certs since last year.

can you please suggest any alternatives to switch to? i hardly can find any alternative which provides free service and is a non-profit org at the same time.

Before this, they were all unencrypted and you had to pay to get a cert. I guess we could go back to that - now knowing that every unencrypted connection is being MITMed (the world is so much more hostile now)...