Cannot delete group where members had a task in the group task course element

RedScreen on bzgbs 10.3.6
2015-10-23 15:27:43,160 [ajp-nio2-8008-exec-154] ERROR DBRuntimeException - OLAT::ERROR ^%^ N8-E768

Mozilla/5.0 (Windows NT 6.3; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/46.0.2490.71 Safari/537.36 ^%^ **RedScreen** commit failed, rollback transaction

<dispatchinfo>
	<componentinfo>
		<compname>ffo_main_group_delete_confirmation</compname>
		<compclass>org.olat.core.gui.components.form.flexible.impl.FormWrapperContainer</compclass>
		<extendedinfo>n/a</extendedinfo>
		<event>
			<class&gt;org.olat.core.gui.control.Event</class&gt;
			<command>validation ok</command>
			<tostring>com:validation ok,org.olat.core.gui.control.Event@d62b3f02</tostring>
		</event>
	</componentinfo>
	<controllerinfo>
		<controllername>org.olat.group.ui.main.BusinessGroupDeleteDialogBoxController</controllername>
		<controllername>org.olat.group.ui.main.SearchBusinessGroupListController</controllername>
		<controllername>org.olat.group.ui.main.OverviewBusinessGroupListController</controllername>
		<controllername>org.olat.core.commons.fullWebApp.BaseFullWebappController</controllername>
	</controllerinfo>
</dispatchinfo>

>>> stack of 1.cause::org.olat.core.logging.DBRuntimeException: commit failed, rollback transaction
-> at org.olat.core.commons.persistence.DBImpl.commit(DBImpl.java:782)
   at org.olat.group.manager.BusinessGroupServiceImpl.deleteBusinessGroup(BusinessGroupServiceImpl.java:765)
   at org.olat.group.ui.main.AbstractBusinessGroupListController.doDelete(AbstractBusinessGroupListController.java:1043)
   at org.olat.group.ui.main.AbstractBusinessGroupListController.event(AbstractBusinessGroupListController.java:426)
   at org.olat.core.gui.control.DefaultController$1.run(DefaultController.java:246)
   at org.olat.core.logging.activity.ThreadLocalUserActivityLoggerInstaller.runWithUserActivityLogger(ThreadLocalUserActivityLoggerInstaller.java:108)
   at org.olat.core.gui.control.DefaultController.dispatchEvent(DefaultController.java:243)
   at org.olat.core.gui.control.DefaultController.fireEvent(DefaultController.java:190)
   at org.olat.group.ui.main.BusinessGroupDeleteDialogBoxController.formOK(BusinessGroupDeleteDialogBoxController.java:97)
   at org.olat.core.gui.components.form.flexible.impl.FormBasicController.event(FormBasicController.java:367)
   at org.olat.core.gui.control.DefaultController.dispatchEvent(DefaultController.java:202)
>>> stack of 2.cause::javax.persistence.RollbackException: Error while committing the transaction
-> at org.hibernate.jpa.internal.TransactionImpl.commit(TransactionImpl.java:94)
   at org.olat.core.commons.persistence.DBImpl.commit(DBImpl.java:736)
   at org.olat.group.manager.BusinessGroupServiceImpl.deleteBusinessGroup(BusinessGroupServiceImpl.java:765)
   at org.olat.group.ui.main.AbstractBusinessGroupListController.doDelete(AbstractBusinessGroupListController.java:1043)
   at org.olat.group.ui.main.AbstractBusinessGroupListController.event(AbstractBusinessGroupListController.java:426)
   at org.olat.core.gui.control.DefaultController$1.run(DefaultController.java:246)
   at org.olat.core.logging.activity.ThreadLocalUserActivityLoggerInstaller.runWithUserActivityLogger(ThreadLocalUserActivityLoggerInstaller.java:108)
   at org.olat.core.gui.control.DefaultController.dispatchEvent(DefaultController.java:243)
   at org.olat.core.gui.control.DefaultController.fireEvent(DefaultController.java:190)
   at org.olat.group.ui.main.BusinessGroupDeleteDialogBoxController.formOK(BusinessGroupDeleteDialogBoxController.java:97)
   at org.olat.core.gui.components.form.flexible.impl.FormBasicController.event(FormBasicController.java:367)
>>> stack of 3.cause::javax.persistence.PersistenceException: org.hibernate.exception.ConstraintViolationException: could not execute statement
-> at org.hibernate.jpa.spi.AbstractEntityManagerImpl.convert(AbstractEntityManagerImpl.java:1763)
   at org.hibernate.jpa.spi.AbstractEntityManagerImpl.convert(AbstractEntityManagerImpl.java:1677)
   at org.hibernate.jpa.internal.TransactionImpl.commit(TransactionImpl.java:82)
   at org.olat.core.commons.persistence.DBImpl.commit(DBImpl.java:736)
   at org.olat.group.manager.BusinessGroupServiceImpl.deleteBusinessGroup(BusinessGroupServiceImpl.java:765)
   at org.olat.group.ui.main.AbstractBusinessGroupListController.doDelete(AbstractBusinessGroupListController.java:1043)
   at org.olat.group.ui.main.AbstractBusinessGroupListController.event(AbstractBusinessGroupListController.java:426)
   at org.olat.core.gui.control.DefaultController$1.run(DefaultController.java:246)
   at org.olat.core.logging.activity.ThreadLocalUserActivityLoggerInstaller.runWithUserActivityLogger(ThreadLocalUserActivityLoggerInstaller.java:108)
   at org.olat.core.gui.control.DefaultController.dispatchEvent(DefaultController.java:243)
   at org.olat.core.gui.control.DefaultController.fireEvent(DefaultController.java:190)
>>> stack of 4.cause::org.hibernate.exception.ConstraintViolationException: could not execute statement
-> at org.hibernate.exception.internal.SQLExceptionTypeDelegate.convert(SQLExceptionTypeDelegate.java:72)
   at org.hibernate.exception.internal.StandardSQLExceptionConverter.convert(StandardSQLExceptionConverter.java:49)
   at org.hibernate.engine.jdbc.spi.SqlExceptionHelper.convert(SqlExceptionHelper.java:126)
   at org.hibernate.engine.jdbc.spi.SqlExceptionHelper.convert(SqlExceptionHelper.java:112)
   at org.hibernate.engine.jdbc.internal.ResultSetReturnImpl.executeUpdate(ResultSetReturnImpl.java:211)
   at org.hibernate.engine.jdbc.batch.internal.NonBatchingBatch.addToBatch(NonBatchingBatch.java:62)
   at org.hibernate.persister.entity.AbstractEntityPersister.delete(AbstractEntityPersister.java:3400)
   at org.hibernate.persister.entity.AbstractEntityPersister.delete(AbstractEntityPersister.java:3630)
   at org.hibernate.action.internal.EntityDeleteAction.execute(EntityDeleteAction.java:114)
   at org.hibernate.engine.spi.ActionQueue.executeActions(ActionQueue.java:465)
   at org.hibernate.engine.spi.ActionQueue.executeActions(ActionQueue.java:351)
>>> stack of 5.cause::com.mysql.jdbc.exceptions.jdbc4.MySQLIntegrityConstraintViolationException: Cannot delete or update a parent row: a foreign key constraint fails (`bzgbs`.`o_gta_task`, CONSTRAINT `gtask_to_bgroup_idx` FOREIGN KEY (`fk_businessgroup`) REFERENCES `o_gp_business` (`group_id`))
-> at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
   at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
   at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
   at java.lang.reflect.Constructor.newInstance(Constructor.java:422)
   at com.mysql.jdbc.Util.handleNewInstance(Util.java:377)
   at com.mysql.jdbc.Util.getInstance(Util.java:360)
   at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:971)
   at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3887)
   at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3823)
   at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:2435)
   at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2582)