Blocked pipe in the bathroom

Maintenance

Hi, all.

We had TT saying that they had water from dishwasher coming out in the bathroom, and now the bathroom is not draining well. When we asked them if they rinse the dishes before putting them into the dishwasher, they said yes. But then how do they know that the water is from the dishwasher, unless they have remnants of food floating in it? Probably, a combination of uncleaned hair and scum and food that gets stuck in it.
We never had this problem before. It took these TT seven months to get it blocked (unless it blocked itself). It's also a modern flat with rather new piping.

Any insight is appreciated. Also, do you think caustic soda may solve the problem? We will try it, anyway. Unfortunately, the previous LL's criticism was that they were not the cleanest TT. Also, do I understand it right that it's their responsibility to pay for plumber if we fail unblocking it ourselves?