I play in Hard (roguelike) mode on MacOS version 10.14.2, ADOM version 3.3.2.
Here are some more details. When I die, I consistently get a message saying "The highscore has been locked for a long time (...)", I can do the following:
[w] wait some more time (ultimately useless)
[q] quit without saving the highscore (oddly tells me "You didn't make it into the highscore. At least 1 point was required." I've had this happen after checking my score was above 1 on the death summary screen)
[f] force saving the highscore (this option consistently gets me the JSON error message)