I can confirm everything that xplosionmind reported.
I set up my main account without using Google services and tried to install Posteid from the Aurora Store, but it doesn't work, which seems normal to me so far.
I also set up a second account where I activated Google services for the apps that require them, and I was able to install Posteid there. I launched it and a notification from Grapheneos appeared, informing me that the app previously used the API. So far, so good, I guess.
Then the same message from xplosionmind appeared inside the app: “An error occurred while starting the app. We recommend that you try again and check the store for any app updates.”
This app is very important in Italy, so I hope the problem gets resolved soon. For now, I am forced to keep my old smartphone in order to use it, as it is an essential app.