Collaborative Editing cannot be enabled after importing a space from Confluence Cloud

2018-05-16 12:13:58,517 ERROR [Long running task: EnableTask] [plugins.synchrony.tasks.AbstractConfigLongRunningTask] runInternal An error occurred when running a Synchrony ConfigLongRunningTask
 -- url: /rest/synchrony-interop/enable | referer: https://confluence.example.com/admin/confluence-collaborative-editor-plugin/configure.action | traceId: 67bfdcb7c0b2beb7 | userName: admin
org.springframework.dao.DataIntegrityViolationException: could not execute statement; SQL [n/a]; constraint [fk_notifications_content]; nested exception is org.hibernate.exception.ConstraintViolationException: could not execute statement
	at org.springframework.orm.hibernate5.SessionFactoryUtils.convertHibernateAccessException(SessionFactoryUtils.java:241)
	at org.springframework.orm.hibernate5.HibernateTemplate.doExecute(HibernateTemplate.java:366)
	at org.springframework.orm.hibernate5.HibernateTemplate.executeWithNativeSession(HibernateTemplate.java:326)
	at com.atlassian.confluence.core.persistence.hibernate.HibernateObjectDao.findNamedQueryStringParams(HibernateObjectDao.java:444)
	at com.atlassian.confluence.core.persistence.hibernate.HibernateObjectDao.findNamedQueryStringParam(HibernateObjectDao.java:320)
	at com.atlassian.confluence.pages.persistence.dao.hibernate.AbstractHibernateAttachmentDao.getLatestVersionsOfAttachmentsWithAnyStatus(AbstractHibernateAttachmentDao.java:134)
	at com.atlassian.confluence.pages.attachments.CachingAttachmentDao.getLatestVersionsOfAttachmentsWithAnyStatus(CachingAttachmentDao.java:138)
	at com.atlassian.confluence.pages.DefaultAttachmentManager.getLatestVersionsOfAttachmentsWithAnyStatus(DefaultAttachmentManager.java:288)
	at com.atlassian.confluence.pages.DelegatorAttachmentManager.getLatestVersionsOfAttachmentsWithAnyStatus(DelegatorAttachmentManager.java:75)
	at com.atlassian.confluence.pages.CachingAttachmentManager.getLatestVersionsOfAttachmentsWithAnyStatus(CachingAttachmentManager.java:67)
	...
	at com.atlassian.confluence.pages.DefaultPageManager.lambda$removeStaleSharedDrafts$10(DefaultPageManager.java:763)
	at org.springframework.transaction.support.TransactionTemplate.execute(TransactionTemplate.java:133)
	at com.atlassian.confluence.pages.DefaultPageManager.removeStaleSharedDrafts(DefaultPageManager.java:761)
	...
	at com.atlassian.confluence.plugins.synchrony.tasks.EnableTask.execute(EnableTask.java:53)
	at com.atlassian.confluence.plugins.synchrony.tasks.AbstractConfigLongRunningTask.runInternal(AbstractConfigLongRunningTask.java:65)
	at com.atlassian.confluence.util.longrunning.ConfluenceAbstractLongRunningTask.run(ConfluenceAbstractLongRunningTask.java:26)
	at com.atlassian.confluence.util.longrunning.ManagedTask.run(ManagedTask.java:52)
	at com.atlassian.confluence.impl.util.concurrent.ConfluenceExecutors$ThreadLocalContextTaskWrapper.lambda$wrap$1(ConfluenceExecutors.java:90)
	at com.atlassian.confluence.vcache.VCacheRequestContextOperations.lambda$doInRequestContext$0(VCacheRequestContextOperations.java:50)
	at com.atlassian.confluence.impl.vcache.VCacheRequestContextManager.doInRequestContextInternal(VCacheRequestContextManager.java:87)
	at com.atlassian.confluence.impl.vcache.VCacheRequestContextManager.doInRequestContext(VCacheRequestContextManager.java:71)
	at com.atlassian.confluence.vcache.VCacheRequestContextOperations.doInRequestContext(VCacheRequestContextOperations.java:49)
	at com.atlassian.confluence.vcache.VCacheRequestContextOperations.lambda$withRequestContext$2(VCacheRequestContextOperations.java:66)
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
	at java.lang.Thread.run(Thread.java:748)
Caused by: org.hibernate.exception.ConstraintViolationException: could not execute statement
	at org.hibernate.exception.internal.SQLStateConversionDelegate.convert(SQLStateConversionDelegate.java:112)
	at org.hibernate.exception.internal.StandardSQLExceptionConverter.convert(StandardSQLExceptionConverter.java:42)
	at org.hibernate.engine.jdbc.spi.SqlExceptionHelper.convert(SqlExceptionHelper.java:111)
	at org.hibernate.engine.jdbc.spi.SqlExceptionHelper.convert(SqlExceptionHelper.java:97)
	at org.hibernate.engine.jdbc.internal.ResultSetReturnImpl.executeUpdate(ResultSetReturnImpl.java:207)
	at org.hibernate.engine.jdbc.batch.internal.NonBatchingBatch.addToBatch(NonBatchingBatch.java:45)
	at org.hibernate.persister.entity.AbstractEntityPersister.delete(AbstractEntityPersister.java:3246)
	at org.hibernate.persister.entity.AbstractEntityPersister.delete(AbstractEntityPersister.java:3483)
	at org.hibernate.action.internal.EntityDeleteAction.execute(EntityDeleteAction.java:99)
	at org.hibernate.engine.spi.ActionQueue.executeActions(ActionQueue.java:582)
	at org.hibernate.engine.spi.ActionQueue.executeActions(ActionQueue.java:456)
	at org.hibernate.event.internal.AbstractFlushingEventListener.performExecutions(AbstractFlushingEventListener.java:337)
	at org.hibernate.event.internal.DefaultAutoFlushEventListener.onAutoFlush(DefaultAutoFlushEventListener.java:50)
	at org.hibernate.internal.SessionImpl.autoFlushIfRequired(SessionImpl.java:1366)
	at org.hibernate.internal.SessionImpl.list(SessionImpl.java:1451)
	at org.hibernate.query.internal.AbstractProducedQuery.doList(AbstractProducedQuery.java:1426)
	at org.hibernate.query.internal.AbstractProducedQuery.list(AbstractProducedQuery.java:1398)
	at com.atlassian.confluence.core.persistence.hibernate.HibernateObjectDao.lambda$findNamedQueryStringParams$5(HibernateObjectDao.java:468)
	at org.springframework.orm.hibernate5.HibernateTemplate.doExecute(HibernateTemplate.java:359)
	... 85 more
Caused by: org.postgresql.util.PSQLException: ERROR: update or delete on table "content" violates foreign key constraint "fk_notifications_content" on table "notifications"
  Detail: Key (contentid)=(4423910) is still referenced from table "notifications".
	at org.postgresql.core.v3.QueryExecutorImpl.receiveErrorResponse(QueryExecutorImpl.java:2455)
	at org.postgresql.core.v3.QueryExecutorImpl.processResults(QueryExecutorImpl.java:2155)
	at org.postgresql.core.v3.QueryExecutorImpl.execute(QueryExecutorImpl.java:288)
	at org.postgresql.jdbc.PgStatement.executeInternal(PgStatement.java:430)
	at org.postgresql.jdbc.PgStatement.execute(PgStatement.java:356)
	at org.postgresql.jdbc.PgPreparedStatement.executeWithFlags(PgPreparedStatement.java:168)
	at org.postgresql.jdbc.PgPreparedStatement.executeUpdate(PgPreparedStatement.java:135)
	at com.mchange.v2.c3p0.impl.NewProxyPreparedStatement.executeUpdate(NewProxyPreparedStatement.java:410)
	at org.hibernate.engine.jdbc.internal.ResultSetReturnImpl.executeUpdate(ResultSetReturnImpl.java:204)
	... 99 more

ERROR: update or delete on table "content" violates foreign key constraint "fk_notifications_content" on table "notifications"

$ grep 'is still referenced from table \"notifications\"' atlassian-confluence.* | awk '{print $4}' | sort | uniq | awk -F"[()]" '{print $4}'
21659807
21659827
21659896
4423910
4423941
4423953
4423966
4423967
4423968
4424797
4424798
4424936

DELETE FROM NOTIFICATIONS
WHERE CONTENTID IN (
  SELECT CONTENTID FROM CONTENT
  WHERE (CONTENT_STATUS = 'deleted' AND SPACEID = <spaceid-here>)
     OR PREVVER IN (
       SELECT CONTENTID FROM CONTENT
       WHERE CONTENT_STATUS = 'deleted' AND SPACEID = <spaceid-here>));