Hi aykanaskit,
Welcome to Apple Support Communities. We understand that you're having an issue with playing videos in Safari. We'd like to help.
When you are trying to play a video, are you doing this from a specific website? Do you see this behavior all all websites? Are the videos streaming or downloaded through Safari?
Let's have you try to isolate this issue. Booting into safe mode will help you see if this might be caused by some login in items: How to use safe mode on your Mac
If you are able to watch your videos successfully in safe mode, restart your computer and try to see if that resolved your issue.
Hope that helps!