Unfortunately, I still haven't a solution for my personnal case.
I'm unable to contact my API on a aws s3 ubuntu 20.04 from my Xamarin app using restsharp (it's working fine with postman or chrome).
I have a Trustfailure error.
I tried this Help thread for DST Root CA X3 expiration (September 2021) - #409 by dmaehler , and the X3 is no more in my SSL path, checked it with SSLabs.
I tried to update ca-certificates 20210119~20.04.2 but i'm not 100% I did it well.
I tried to use cert-sync (if I understand well, mono use is own certificates and this link them with ubuntu's ?)
I don't know what I can do, if you have any clue