When using the Scrum board inline + Create under a parent issue to create a sub-task, Jira opens the create dialog without a project selected. The dialog immediately displays "The issue type selected is invalid". If a project is manually selected, Jira opens the standard issue creation form (Story/Bug/Task etc.) instead of the Sub-task form. Network response contains:
Creating sub-tasks from within the parent issue works normally. The issue affects all users and multiple browsers.