One of the largest certificate issuers Let’s Encrypt had their root certificate validity end on 30. 9. 2021. In case your validity has not been renewed, an error might occur with eM Client licensing and other programs as well.

How to check your certificate validity in Windows?

1. Navigate to the Start menu in Windows and type certmgr
2. Open the Certificate Manager app by pressing enter and allow changes
3. In the left window navigate to Third-Party Root Certification Authorities
4. Click on the file Certificates that shows up
5. Go to the right side and find ISRG Root X1 in the ‘Issued to’ column- it is ordered alphabetically
6. Check the expiration date in the third column

Now, in case your certificate has validity to 30.9.2021 it means it is past due. 

Here is how to solve the problem.

1. Download the attached file 'ISRG Root X1.cer' and open it on your PC
2. A window pops up, click on Install Certificate
3. Click Next on the following screen
4. Check the option to Place all certificates in the following store
5. Click Browse and select Trusted Root Certification Authorities and click OK
6. Click on Next and then Finish to import the certificate