NPE when reading map from DB

Greetings,

this NPE happened in our app first time today while reading objects from DB.

Could u please check what could be the issue ?

Inner message:
Cannot invoke "com.objectdb.o.ORS.YD()" because "<parameter1>" is null
--------------------
com.objectdb.o.TYR.<init>(TYR.java:113)
com.objectdb.o.ENT.newTypeReader(ENT.java:1525)
com.objectdb.o.MPT.readArray(MPT.java:404)
com.objectdb.o.MPT.r(MPT.java:333)
com.objectdb.o.TYT.extractMap(TYT.java:109)
com.objectdb.o.ENT.extractMap(ENT.java:1608)
objectdb.java.util.HashMap.__odbBeforeAccess(Unknown Source)
objectdb.java.util.HashMap.keySet(Unknown Source)
quant.phdsc_new.utility.Tool.deepCopy(Unknown Source)

Maybe this is caused by accessing an object (a persistent map field according to the stack trace) when the connection to the database is closed.