Why Let's encrypt SSL certificate not working from 30 Sep 2021 in some (client) PC?

We are using the Let's Encrypt SSL certificate from 2020 and it's working fine till now, But since the morning of 30 Spe 2021 our some clients facing "Site not secure" SSL certificate error but the same site not showing any SSL error in other PCs.

Summary: Same website working fine on some PC and same website showing SSL error "Site not secure" in other PC.

So timely I am using Paid SSL and it's working fine on all PC. So the question is What is the issue with Let's Encrypt SSL and When it will be solved & should we use it for our important website?

Thanks.