Videos not playing, or glitchy, or sometimes saying “Video cannot be played?” Some subscription-based streaming services use Microsoft Silverlight, which hasn’t been updated for High Sierra; it’s only certified for 10.6 through to 10.11. For a start, install the latest version and see bit.ly/check-silverlight if it won’t install. As of High Sierra, Silverlight can’t access local files by default, which may include downloaded/buffered videos.
In Safari > Preferences > Websites, select Silverlight on the left, then Alt-click the pop-up menu next to the problematic website and disable “Run in Safe Mode.” Confirm when asked. This may do the trick, but note that doing this presents a security risk.
Increased browser security in Safari in High Sierra means that older plugins such as Silverlight may run into trouble — there is a workaround, though.