An SSL certificate has been successfully deposited on the website, but the website is still considered insecure. This is usually due to mixed content, i.e. a mixture of secure and insecure content.
Solution: In general, change all existing links from http to https. However, if the faulty link cannot be found, the use of Google Chrome can help. Here you click on the right mouse button on the corresponding page, then on Investigate -> Console. The exact element is then displayed and quick troubleshooting is possible.