Updated to 5.1.14.
Bootstrap 4 were already in use.
Cache cleared on Chrome.
That does NOT had solved the problem .
Unmasking the download link SOLVED my immediate problem since my files can be seen only by loggedin users.
BTW: Italian translation of the unmask link option titles are completely wrong. I suggest to change it to:
OPTION TITLES: Link di download mascherato / Link di download pubblico
OPTION HELP TEXT: can remain unchanged.
