A certificate can be incorrect because of many reasons.
One of the experienced reason is that the certificate has an unknown critical extension.
The browser gives the following error:
1. Generate a new certificate
2. Copy the new certificate in PEM format to the directory /mnt/firmware/root
3. Log in to the core-shell
4. Copy the new certificate over the old one
cp /root/NEW_CERTIFICATE /etc/lighttpd/server.pem
5. Restart web service
systemctl restart lighttpd
6. Log in to web user interface
7. Upload the new certificate again at Basic Settings --> Management --> SSL certificate --> Server X.509 certificate
8. Press Commit.
9. To test the new certificate, open a different browser and check if the authentication form appears correctly and can log in.