Hi! We are sorry you have this issue, the matter can be either you have something wrong with HTTPS, or the provider / proxy replaces the certificate and our website doesn’t accept it. Our devs are checking it already on our end, we will get back as soon as we have the news on that issue. Thank you.
Unfortunately, we were not able to reproduce this issue on our end, it seems that the matter is not in our SSL-certificate or app. We can assume that the thing may be that your computer was unable to sync the system time using the time servers. You need to try to sync the time manually. Another reason that could cause this issue may be IP address forwarding.
Please, kindly check these points on your end and tell us if that helped! Thank you.
I’ve checked everything, even the request / response of the program responsible for the login (https://api.icons8.com) I’ve disabled the firewall, installed third-party certificates, even applied ssl unpinning methods but no results, I bought the premium version In order to have all icons released, I would like you to develop a license system if the same happened, I could insert my license and use the other blocked functions of the program. Oobs: I would like to know which request is used for authentication, whether socket / post / get, among others …
I’m sorry the issue still exists 
We have checked everything on our end again, but we have our certificate working well, therefore we can only assume you have some issues with the internet provider or network we cannot help with however much we want. Could you try to login from another device and/or another provider (better to do both)?
If that wouldn’t help, the last possible reason can be that your OS does not support the new TLS options required on our server. Therefore we may recommend to update the OS to the newest version and download our app from Windows Store if you haven’t yet.
And as for the request the app uses for authentication, it is https://api.icons8.com/api/auth/login