My manager was unable to create an Epic from the Plan form in an Agile board. Using the create Epic link, the window successfully popped up, however it defaulted to "custom fields" rather than "all". We had a required field that was not visible, and he was unable to see the field, therefore could not create the ticket. Yes, he could have clicked All (If he had known that was how JIRA worked). I verified that this is specific to clicking "Create Epic" from the plan page, and not the Create Issue page.

Reproduction steps:
1) access any agile board that uses epics
2) click "Epic" on the left side
3) scroll over "Epics", and click the link that appears "Create epic"
4) click "Configure Fields" and notice "Custom" is selected, not "All"