corresponding to your suggestion I created a private ticket in reply to http://www.objectdb.com/database/forum/1109.
Attached you will find my database file (I removed most of the content but one corrupt instance is still included).
I error happens when reading the field "Event.participants" of the entity with id "a1bef640-2191-11e6-942d-485ab6c92d08". The explorer just shows an ArrayList of length 0 but this isn´t the case. Since the type "Participant" is correct there must be some issue with the database file (before deleting all other entities of type "Event" I could not read the field "Event.participants" from any of them).
Thanks in advance