The reason you are getting this error is because your SSL Certificate may be corrupted. Rebuild it by executing the following 5 commands in the order provided.
Code:openssl genrsa -des3 -passout pass:yourpassword -out /root/ispconfig/httpd/conf/ssl.key/server.key2 1024
Code:openssl req -new -passin pass:yourpassword -passout pass:yourpassword -key /root/ispconfig/httpd/conf/ssl.key/server.key2 -out /root/ispconfig/httpd/conf/ssl.csr/server.csr -days 365
Code:openssl req -x509 -passin pass:yourpassword -passout pass:yourpassword -key /root/ispconfig/httpd/conf/ssl.key/server.key2 -in /root/ispconfig/httpd/conf/ssl.csr/server.csr -out /root/ispconfig/httpd/conf/ssl.crt/server.crt -days 365
Code:openssl rsa -passin pass:yourpassword -in /root/ispconfig/httpd/conf/ssl.key/server.key2 -out /root/ispconfig/httpd/conf/ssl.key/server.key
Code:chmod 400 /root/ispconfig/httpd/conf/ssl.key/server.key
Now restart your ISPConfig
Code:/etc/init.d/ispconfig_server restart