The correct action in this case is to update the browser or the operating system. Certificate warnings may occur if the browser or the operating system is outdated and does not have the latest root certificates or does not recognize newer encryption methods. An outdated browser or OS may not recognize newer certificates from legitimate websites, hence throwing a warning.

Installing third-party certificates manually could introduce security risks if the user is not careful about the source, therefore it is not the best initial action. Disabling security warnings will not solve the issue and can lead to security risks by not alerting the user to potentially dangerous sites. Reissuing the certificate is not necessary if the certificate is already valid and recognized as such on updated systems.