i'm Korean who struggling with the usages articles in English.
On cambridge dictionary, It describe one of the meanings of "start-up" as followed.
start-up: the act or process of starting or making something start
and as for "start"
start: the beginning of something, the act of beginning to do something
to me, those two are kinda the same.
"Once you have disabled the program, reboot the computer to see if its absence causes a problem on startup." it's an example from the dictionary.
i would say like "~ on the startup" because, 'startup' is quite specific in the context. Like "the startup of the program when you've disable the program"
as i understand, "the" as in "from the start" is used because of the same reason.
I don't want you guys saying "it's just a phrase" or "that's the way it is". :(