Can't login to panel after migration

I tried this solution:

But none of the sulutions worked for me.
I get this error:

I also deleted all users, and enabled the function to add a new one via config.
When I create a new one I get this:

Really strange because the dialoge box is empty?

Is it possible that this has to do with the hosting/server of my client?

On the same server I uploaded the plainkit from Kirby.
I added the config to add a first user. Same problem, empty dialog.

Any idea what can go wrong?

Have you cleared the browser cache and deleted sessions in the browser and the site/sessions folder?

Yes I did, do you think it can be something that has to do with the hosting of my client? I published 10+ kirby sites, and this never happened to me.

Hm, this empty dialog came up before here: Lokal kein Zugang zum Installer, Panel und Login (de)

But I have no idea if it is in any way related.

Who is the hosting provider?

@texnixe The hosting is your-webhost [dot] nl
I’m in contact with the host. I will update when I have more information.

The hosting contacted me and told me that it had something to do with “mod_security”.
They Fixed it for me.