Version 2.8.4 fixes a critical bug that in rare cases may cause collision between different schema versions of entity classes, when schemas have changes many times (usually more than 20 times).
This bug mainly affects queries on hierarchy of multiple entity classes (i.e. a query on a super class that contains several subclasses), where an unexpected exception or unexpected query results are possible (although very rare). It may also cause a failure in updating indexes in some very specific cases.
The data in the database is not expected to be affected by this issue.
ObjectDB - Fast Object Database for Java (JPA/JDO)