Error: Call to a member function getUntranslatedString() on string

I can confirm that the issue was originating from custom code in our case. We can fix it there.

The plugin in my case is the gnode groupnode contentenabler.
The label is a string, the description is a TranslatableMarkup, for some reason.

For clarity, this is what I did to cause this error:

In a custom Deriver class, getDerivativeDefinitions method, I changed this:

      $this->derivatives[$name]->set('label', t('Organization site (@type)', ['@type' => $label]));

to this:

      $this->derivatives[$name]->set('label', t($label));

Instead of doing logic checks, I think it would be simpler to create a setLabel() method so that you could force the parameter to be TranslatableMarkup. There is already a getLabel() method.