The problem reappeared and persisted since. Fortunately, I didn’t have to use related functionality extensively and what I had to do I did in the Cloud.. waiting for the better 10.1 version.
Now I face the same problem and I wish to know if it is possible to remedy it.
A clean start doesn’t help.
And the problem seemed to be related to that.
For instance, CountryData["EU"] resulted in 28 unevaluated Entity["Country",~].
I understood that it should have been a server issue after I have defaulted everything and cleaned all AppData files created by Mathematica and those which remained from the previous versions.