When you have one Story in one project linked to an Epic in another project and this Story is moved to the Epic's project and the Story issue type is also moved to Epic, its link to the original Epic is not cleared during the move wizard and the link can not be removed from the edit screen as well. The following message appears in the Epic Link field:

An epic cannot have another epic linked to it.

Steps to reproduce:

1. Create an Epic in one project
2. Create a Story in another project
3. Link this Story to that Epic
4. Move this Story to that Epic's project changing its type to Epic too

Workaround:

Edit the Story's (now Epic) issue type back to Story, remove the Epic Link, then edit its type back to Epic.