Fortunately it still is easy to find the solution for non-playing YouTube content on Chromium from the relevant questions here but closing of these questions led to an unfortunate constellation:
Chromium browser not loading youtube video
is closed as a dupe from question 2. where the answer links back to 1.
Can't watch youtube videos with chromium on ubuntu 13.04 64-bit
links to 1. in the most upvoted answer but the question is closed too, it is a dupe of another question:
Youtube says "This video is currently unavailable"
but this was closed as NARQ.
So we now have a maze of 3 closed questions for this issue. Question 1. has the best answer. Question 3. has the best title for googling but an answer that does not always work.
Let me therefore suggest to