When your certificate is expired, you need to replace it on the Clientless RDP Gateway. Please make sure to rename the new certificate name to clientlessrdpcert.pfx.

Open a session to the Clientless RDP Gateway and run the following commands:

cd /etc/tomcat8
sudo rm -r clientlessrdpcert.pfx
sudo wget urltocertificate -O clientlessrdpcert.pfx
sudo nano server.xml

Search for the connector listening on 8443. Scroll to the right and update the password.

sudo service tomcat8 restart