In a test server I was given access to by the kindness of its owner, I tried adding my own UID to the owner's:
admins=<owner_UID>,<my_UID>
as a consequence, neither of us was any longer recognised as an admin; when I put my UID only, I was recognised as an admin.
Shall we deduce that the "admin" line of the server.properties file only accepts one admin UID?
P.S.: I searched the fora for a similar topic, but I didn't find any; either my search skills suck or the issue is new and possibly related with the change from player names to UID...?