Your username implies you're a mainland European, maybe a German. I think your prospective employer has done you a big favor by checking your teaching out before unleashing you on the students, who will be on summer vacation at this time, in any case. It's better for you to part company with your university now than be forced out four or six weeks into the new semester, at which time it'd be damned difficult for you to get a new position.
I'm guessing that that university has professors who are accent snobs. They feel qualified to judge English speakers' accents while at the same time having lousy English accents themselves. It's not good that they have enticed you there from afar only to reject you. And that's after you've gotten a signed contract from them, and have not even been given a chance to teach their students! That's how badly we foreigners can be treated by a prospective employer in China. In a word, many Chinese employers cannot be trusted. Another university that operates in this shabby way is Linyi University of Shandong province. I think Wuhan Polytechnic University and Linyi University have much in common and should both be placed on a black list!