How to prove this statement, and how to prove this malicious intent of the owners of the exchange, and not the technical or software particular qualities
of the equipment?
You don't need to prove anything, leave it up to investigators in charge of Upbit's case.
I can't think any sort of illegal activities that an exchange can do except for market manipulation and stealing customer funds.
They probably haven't tried stealing customer funds nor market manipulation, it's just that I think they are taking the matter personally. They might have been alarmed to large volumes of crypto trading and many profits made by crypto users from it. the statement that made me think that they are really acting based on the subjective opinion of their government official is this:
The barrage is alarming official including Prime Minister Lee Nak-yon. He says that cryptocurrency might corrupt the nation’s youngsters.