Timer Event is not working

i am using a timer to wait one particular task. but when timer is getting executed i got the error class not found. getting this error only when timer event is fired. rest of my workflow is working fine.

please help me!

here is log:

org.camunda.bpm.engine.ProcessEngineException: Cannot deserialize object in variable 'commonBO': ENGINE-09017 Cannad class 'com.att.comet.bpm.common.modal.CommonBO': ot locom.att.comet.bpm.common.modal.CommonBO
at org.camunda.bpm.engine.impl.variable.serializer.AbstractSerializableValueSerializer.readValue(AbstractSerializableValueSerializer.java:85)
at org.camunda.bpm.engine.impl.variable.serializer.AbstractSerializableValueSerializer.readValue(AbstractSerializableValueSerializer.java:31)
at org.camunda.bpm.engine.impl.persistence.entity.util.TypedValueField.getTypedValue(TypedValueField.java:96)
at org.camunda.bpm.engine.impl.persistence.entity.VariableInstanceEntity.getTypedValue(VariableInstanceEntity.java:269)
at org.camunda.bpm.engine.impl.core.variable.scope.AbstractVariableScope.collectVariables(AbstractVariableScope.java:114)
at org.camunda.bpm.engine.impl.core.variable.scope.AbstractVariableScope.collectVariables(AbstractVariableScope.java:121)
at org.camunda.bpm.engine.impl.core.variable.scope.AbstractVariableScope.collectVariables(AbstractVariableScope.java:121)
at org.camunda.bpm.engine.impl.core.variable.scope.AbstractVariableScope.collectVariables(AbstractVariableScope.java:121)
at org.camunda.bpm.engine.impl.core.variable.scope.AbstractVariableScope.getVariablesTyped(AbstractVariableScope.java:89)
at