What should I do?

I am right now middle of a process with a german company for a sales role. I have done two rounds and they have invited me to an onsite interview. During my interview with the hiring manager she told me that the next step is an onsite interview where I present a case of some sort. But iwhen I get the email from HR nothing was mentioned about this.

It should be said that HR been extremely slow this whole process. Every step and email I send takes a week for them to respond to. At least. For example, in the email regarding the onsite interview they asked about availability for a few dates. I responded to that quite promptly but it took 5 working days for them to answer. I know it is holiday times and that is probably the reason. But then every step and email takes 1-2 weeks, it prolongs it quite a while.

So when I ia week ago recieved the onsite confirmation with times and date from HR I responded that the time suits me well and I also asked that I assume I will get information about the case in a separate email? That was 8 days ago. Still no response. So now I am in the situation where my onsite interview is coming up monday next week and I still dont know whats going on. Do I need to present anything or not? So to avoid this, I called their main line but hiring manager was not there since she is on vacation, so I wrote the hiring manager on linkedin 2 days ago also, but I have no response there either.

So what do I do now? Imagine if I show up and they expect a presentation? It all just wierd.