For this problem: there was a pending software update to the Matrix server that hopefully fixes this (I have installed it now). At least I can log in again using the current live version of the Riot chat client (riot.im/app/) on a desktop computer.
Could you try again please, @inge and @natalia_skoczylas and @fsimonov? You may have to first use the “Logout” function to force a new login attempt.
(If the login does not succeed and a “Unable to connect to Homeserver. Retrying…” message is displayed: that can be a different issue in cases where you’re running low (<1 GiB) on free hard disk space. Try the login in incognito / private browsing mode then.)
The certificate warning is unexpected (nothing changed on our side) but I have a theory. There was a SSL certificate change of the riot.im server’s certificate recently. On my side I see yet another one than the one Natalia mentioned, but that is possible because Riot uses Cloudflare, and Cloudflare has many certificates for its many subdomains which it somehow uses to work against possible DDoS attacks.
For the web version, the Cloudflare certificate changed on 2019-07-11, but they might have rolled out / installed the certificate a few days after obtaining it, so that could still fit for the timing that Natalia has observed. And it would explain why the app still works while the browser version (which uses Cloudflare) had issues. Because the app does not use Cloudflare:
if you don’t want to rely on Cloudflare’s availability to access a Riot client, you can download the electron client, or a mobile app, or host your own Riot web instance, or use one hosted by somebody else (source)
So I think there is no danger implied in that certificate warning. To be sure, I would however need to know what server the certificate change is about (as that is not mentioned in the error message Natalia got). So, @natalia_skoczylas, could you visit https://riot.im/app/ and click on the lock symbol in the browser’s address bar and copy & paste the values of “Common Name (CN)” and “SHA-256 Fingerprint” of the certificate of that site? Let’s see if it is the same as the one you go the error message for. If so, and if it’s by Cloudflare, everything is fine.