Yandex Browser no longer supports the Unity Web Player plugin and the NPAPI technology it's based on. More information about the cessation of further development of NPAPI on the Unity company website: http://blogs.unity3d.com/ru/2015/10/08/unity-web-player-roadmap/

Try clearing cookies by pressing Ctrl+Shift+Del (Windows) or ⌘ + Shift + Del (Mac OS). In the popup window, flag 'the beginning of time’ in the drop-down menu next to ‘Obliterate the following items from’ and choose ‘cookies and other site data’ -> Click ‘Clear browsing data'.
See if it worked at https://zen.yandex.com/demo.
If it didn’t, try changing your browsing profile or making a new one in Settings. To change your browsing profile, go to Settings by clicking the sandwich icon in the top right-hand corner of your browser -> Scroll down to ‘User profiles’ and choose another user profile and make it ‘current’. Or click ‘Add profile’ -> Choose a picture and a name -> Click ‘Add’ and make your new profile ‘current’. See if this worked at https://zen.yandex.com/demo.

You need to update the browser. Open the browser Menu, go to Advanced, and press About Yandex Browser. Look for the Update button under the Yandex Browser logo. After the update installs, restart the browser. If you encounter any issues, copy the error message and send it to us.
If you're using Linux, download the latest version of Yandex Browser from the repository or from https://browser.yandex.com/download/?linux=1.

If Browser freezes or functions slowly, write to Support. Describe, in as much detail as possible:
  - What actions cause Browser to freeze
  - What took place before this issue appeared (for example, Browser was reinstalled, an application or extension was reinstalled or updated, etc)

If Browser opens pages slowly, please describe the following in your message to Support:
  - Does this problem occur for all pages, or only specific ones (if yes, then please specify which pages)
  - Please try to collect the net-internals log by following these instructions, upload it to the cloud storage service of your choice, and send us the link
  - If errors occur when opening pages, please include the error message text in your message.
This information will help us identify and solve the problem more quickly.