Could you please verify that this setting is in the correct objectdb.conf file?
As explained on this page (with more details):
The value of $objectdb (the ObjectDB home directory) is derived from the location of the objectdb.jar file. It is defined as the path to the directory in which objectdb.jar is located, with one exception - if the name of that directory is bin, lib or build the parent directory is considered to be the ObjectDB home directory ($objectdb).
... by default the configuration file is loaded from $objectdb/objectdb.conf.
You can add an XML error to the objectdb.conf file. If your application is still running then it is not the right file.
ObjectDB - Fast Object Database for Java (JPA/JDO)