• You might be using Internet Explorer or any other unsupported browser to access Web Console.
  So first we need to check if the browser is supported by KSC. Ref : https://support.kaspersky.com/KSC/13.1/en-US/96255.htm.
• You may use unsupported certificate's format.
  KSC Web Console can only work with PEM, not PFX format - https://support.kaspersky.com/KSC/13/en-US/191451.htm.
  If you use incorrect format, try to convert the certificate like described here: https://support.kaspersky.com/KSC/13/en-US/201428.htm. After converting the certificate you should be having 2 file formats ready: .crt and .pem.

If you use correct browser and certificate, follow these steps:

1. Run the Web Console installation package (KSCWebConsoleInstaller.12.0.<build number>.exe) again and follow the instruction here to upload the certificate and the key https://support.kaspersky.com/KSC/13/en-US/184363.htm.
   
   
2. During the procedure when setting up the Trusted Administration Servers, you would be requested to provide the path for the trusted KSC's Administration Server certificate. Do note that this is a different certificate from the Web Console's certificate. 
   By default, the Administration Server certificate file is stored in the ALLUSERSPROFILE%\Application Data\KasperskyLab\adminkit\1093\cert folder (ensure you correctly identify the path and also that the path is accessible).
   
3. Once done, open the browser -> clear cache, cookies & history -> exit browser -> Open again. This should solve the problem.