IPE is entirely broken

I had the same problem and got the following error:

PHP Fatal error:  Declaration of Drupal\panels_ipe\Form\PanelsIPEBlockContentForm::actions(array $form, Drupal\Core\Form\FormStateInterface $form_state) must be compatible with Drupal\block_content\BlockContentForm::actions(array $form, Drupal\Core\Form\FormStateInterface $form_state): array in /Library/WebServer/Documents/dedicon-vookam/web/modules/contrib/panels/panels_ipe/src/Form/PanelsIPEBlockContentForm.php on line 18

So fixing the function declaration solved the problem for me.