The supplier has incorrect meter numbers in the system - what to do?

Of course, it could also be the case; just ask the [Messestellenbetreiber] or the installer. That is certainly more effective than letting the internet guess.

Of course, it could also be the case, but why don't you ask the metering point operator or the installer? That is certainly more effective than letting the internet guess.

But the internet has already helped me a lot, because now I know that such a thing even exists.
If the metering point operator is again Pfalzwerke Netz, then unfortunately I cannot ask them directly. They have a hotline setup where the people who answer the phone have absolutely no idea, they just enter data and then say "I will forward this, someone will get back to you."
But I have addressed the installer directly about it, though I am still waiting for a response. They are always very busy.

I would also first clarify the meter number, then try to switch, but in any case, calculate a realistic advance payment and inform the supplier that you consider amount X appropriate. With my supplier, I can adjust this online in my account.

You can also try contacting your network provider. Google "Netzbetreiber suche" and then the first result for me is ovag.

For me, the local municipal utilities do not supply but something outsourced. A few kilometers away it is then Westnetz. They should be knowledgeable and able to help better than the municipal utilities (actually).

We have 2 meters, and Pfalzwerke wants a hefty €390 advance payment in total.

Can't that be reduced? EnBW also set quite high advance payments for me, but I was able to reduce them to about half via the web portal. Otherwise, I would escalate the issue to the arbitration board of the network agency in parallel.

First, it would be helpful to know what is installed with you.

Classic: One meter for the heat pump, one for the household. Both come from the grid and have a meter fuse. After the meter, it goes to the respective consumers. Advantage: You can read everything directly, i.e. "Consumption 1 kWh" on the heat pump meter also means 1 kWh consumption of heat pump electricity. Accordingly, the entire reading setup is simple.

Cascade: The electricity first goes through the main meter and then both to the heat pump and the household meter. The main meter is fed from the grid and has a meter fuse, the household meter is fed from the main meter and has no meter fuse. Advantage: If you have photovoltaics, you can supply both the household (because it is directly connected) or the heat pump (then it runs through the household meter as feed-in but not through the grid). Therefore, it sometimes runs as "Power-to-heat", which is strictly speaking nonsense, but it allows you to use the photovoltaic power for heating without the expensive detour through the grid. However, only the household electricity is directly readable (household meter consumption) and the heat pump electricity must be calculated from (main meter minus household meter). The grid operator does this and reports the consumption to the electricity provider.

Who can say that? The electrician must have wired it this way and the grid operator must be present and seal it. So "Classic or Cascade" is something that cannot be simply switched overnight via web portal but has to be rewired and resealed.

What to do? I would simply write an email or a letter (!) pointing out that the meter numbers in the letter do not correspond to the installed meters and request correction with a deadline. Only if they do not explain or correct this satisfactorily would I escalate.