SSL certificate / HTTPS encryption problems troubleshooter

We have successfully installed ConfTool on our local server, but the SSL certificate that activates the HTTPS protocol which in turn allows for a secure, encrypted connection between browser and website does not seem to work properly.
For instance the browser shows a warning that the connection cannot be trusted or that the encryption of the transferred data is not complete.
How can we solve this problem?

If the HTTPS encryption does not work properly, there can be a number of reasons. To find out what could be the problem, try these different steps:

• Firstly, click on the padlock symbol in the address bar, which will give you more information about the problem at hand (see image 1).
• Secondly, you should check if the installation and set-up of the certificate has been done properly:
  Run an SSL checker or SSL diagnostic tool, e.g. DigiCert® SSL Installation Diagnostics Tool or SSLChecker or the SSL Server Test

• Thirdly, sometimes you might have included errors in the code of CSS or PHP files of your ConfTool installation that let the browser determine that not all elements of the webpage are transferred securely:
  To find out the origin of this problem, open the developer console of your browser.
  For Firefox, go to Tools => Web Developer => Web Console (see image 2)
  The web console will check your code for errors. Take a look at the results and try to solve the problems. In the image you can see that the error message is produced because some display content is loaded from an unencrypted source, leading to a mixed content warning (see image 3).