Uniform: Invalid input for prop "from", expected string or "Kirby\Cms\User" object or collection

I’m currently updating my site from Kirby 2 to Kirby 3.
Most of the stuff worked well thanks to the upgrade guide but now i’m struggling with the Uniform plugin.
I tried to migrate a simple contact form from Uniform 3 to Uniform 4. When I post the form i’m getting the following error:
Invalid input for prop “from”, expected string or “Kirby\Cms\User” object or collection

My controller looks as following:

<?php

use Uniform\Form;

return function ($kirby)
{
$form = new Form([
    'name' => [],
    'email' => [],
    'telephone' => [],
    'contacttype' => [],
    'message' => [],
]);

$siteUrl = $kirby->site()->url();

if(strpos($siteUrl, 'localhost') == false && strpos($siteUrl, '.local') == false && strpos($siteUrl, 'staging') == false): 
    $emailTo = $kirby->site()->adminEmailTo();
else:
    $emailTo = $kirby->site()->adminStageEmailTo();
endif;

if ($kirby->request()->is('POST')) {
    $form->emailAction([
      'to' => $emailTo,
      'from' => $kirby->site()->adminEmailFrom(),
      'subject' => t(getCurrentSite() . '-contact-request'),
    ])
    ->logAction([
      'file' => kirby()->roots()->site().'/messages.log'
    ]);
}
return compact('form');
};
?>

When I hard code the values for from and to, then the email gets sent correct.
However, when I echo $emailTo or $kirby->site()->adminEmailFrom() infront of the emailAction, i get the proper values in there.

Any ideas where this breaks?
Thanks in advance!

Try

$kirby->site()->adminEmailFrom()->value()

Works perfectly, thanks a lot!
With kirby2 ->html() always used to work for me. Did that change?

No, the html() method still exists. But you didn’t use that method, and we want to see a string here.