Update your browser.
If this is no longer possible, the OS X operating system must be updated on the computer.
If an update to the version "Sierra" is no longer possible due to an older device, please try to update to the version in between (El Capitan). Here you can find instructions: https://support.apple.com/de-ch/HT206886
If this is also not possible, we would ask you to switch to the Firefox or Chrome browsers - these run the streams without problems. If your device can no longer install the latest version, you may be able to install older versions - e.g. here: https://www.mozilla.org/en-US/firefox/56.0.1/releasenotes/