Strictly speaking the acsprset.spr itself is not from 3.5.1, as it was saved in 2016 last time, but current version is supposed to open older spritefile too.
EDIT: the error message "Failed to open spriteset file nnn" means that the file could not be opened at all (so it's not because of the bad contents).
Is the file actually in the folder after you unpacked it? What it's size?
Do you open other projects normally?
EDIT: the error message "Failed to open spriteset file nnn" means that the file could not be opened at all (so it's not because of the bad contents).
Is the file actually in the folder after you unpacked it? What it's size?
Do you open other projects normally?