Youtube keeps buffering on all browsers (MacBook Pro)

Dear people

I have been experiencing a strange youtube issue on my Macbook Pro for a week already. Videos keep buffering but not loading at all - just a black screen with the buffering symbol/spinning circle.

• It' relevant to all browsers - tried Safari & Google Chrome
• Similar websites are like Vimeo, for example, are running without issues
• I did most of the troubleshooting like general resets, cookies & history resets, re-install, flash & all updates but it seems that it's not browser related.

Also sharing some hints that I am thinking that are in the core of the issue but can't understand how they are having an influence.

• After Macbook restart Youtube is working for 5 min. Then the issue with the endless buffering appears.
• Youtube is working in safe mode uninterruptedly.

Thanks in advance. I would appreciate any kind of feedback.