We are using OnDemand.

• Editing of the "Epic link" field is not possible anymore.
• Using the plus sign at the top of the "Issues in epic" creates a ticket outside the epic.

Others are experiencing the same change:
https://answers.atlassian.com/questions/18809498/epic-link-field-became-disabled
https://answers.atlassian.com/questions/18450067/unable-to-choose-epic-link-on-issues