Can't create second group of the first group type.

I found a similar issue can occur in \Drupal\group\Entity\Form\GroupContentForm::complete, where a problematic form state is saved in private tempstore (key_value_expire table). The user cannot save the form, nor can they create another node of the same type in the same group for 7 days (at which point the problematic row is automatically deleted from key_value_expire).

In order to permanently fix the issue, you can patch the module to move this bit of code:

$store->delete("$store_id:step");
$store->delete("$store_id:entity");

Above this bit of code:

$form_object->save($form, $form_state);

I won't post a patch because this is technically a different issue. Plus this version of group is almost EOL anyways.