Cannot add Landing page translation via action bar button

Cannot add Landing page translation via action bar button "Add new translation".

Prereq.: Two languages and two siteaccesses.

Steps:

1. Create LP in first language.
2. Switch to second siteaccess.
3. Click "Add new translation" on action bar.
4. Select first language in "Base this translation on...".
5. Click Create on on "Create new translation" modal.

Actual: Nothing happens, error occurs in console "An invalid form control with name='add-translation[language]' is not focusable.".

Remarks: Using the "3 dots" menu and "Add translation" works.