Documentation: Added X509 key values mismatch error section

doc/administration/troubleshooting/ssl.md

@@ -46,6 +46,44 @@ After configuring a GitLab instance with an internal CA certificate, you might n
 
 If you have the problems listed above, add your certificate to `/etc/gitlab/trusted-certs` and run `sudo gitlab-ctl reconfigure`.
 
+## X.509 key values mismatch error
+
+After configuring your instance with a certificate bundle, NGINX may throw the
+following error:
+
+`SSL: error:0B080074:x509 certificate routines:X509_check_private_key:key values mismatch`
+
+This error means that the server certificate and key you have provided do not
+match. You can confirm this by running the following command and comparing the
+output:
+
+```shell
+openssl rsa -noout -modulus -in path/to/your/.key | openssl md5
+openssl x509 -noout -modulus -in path/to/your/.crt | openssl md5
+```
+
+The following is an example of an md5 output between a matching key and certificate. Note the
+matching md5 hashes:
+
+```shell
+$ openssl rsa -noout -modulus -in private.key | openssl md5
+4f49b61b25225abeb7542b29ae20e98c
+$ openssl x509 -noout -modulus -in public.crt | openssl md5
+4f49b61b25225abeb7542b29ae20e98c
+```
+
+This is an opposing output with a non-matching key and certificate which shows different md5 hashes:
+
+```shell
+$ openssl rsa -noout -modulus -in private.key | openssl md5
+d418865077299af27707b1d1fa83cd99
+$ openssl x509 -noout -modulus -in public.crt | openssl md5
+4f49b61b25225abeb7542b29ae20e98c
+```
+
+If the two outputs differ like the above example, there is a mismatch between the certificate
+and key. You should contact the provider of the SSL certificate for further support.
+
 ## Using GitLab Runner with a GitLab instance configured with internal CA certificate or self-signed certificate
 
 Besides getting the errors mentioned in