Question's owner not receiving email notification about new answer posted

Summary

Question's owner is not receiving any email notification regarding new answers being posted on the question.

Environment

• Confluence 1000.340.0
• Questions for Confluence Plugin 2.4.14-D20160804T002212

Steps to Reproduce

1. Create two users (UserA and UserB)
2. Create a test laber (Label1)
3. As UserA watch the Label1
4. As UserA create a question with Label1
5. As UserB post an answer to the question raised in the previous step

Expected Results

UserA should get email notification regarding the answer posted to his question.

Actual Results

• No email notification is sent.
• Error in logs:

  2017-01-13 12:29:45,253 WARN [http-nio-8090-exec-3] [atlassian.confluence.cache.TransactionalCacheFactoryCleaner] lambda$cleaner$2 Forcibly unbound thread-local transactional caches prior to request execution: [com.atlassian.confluence.security.CachingSpacePermissionManager.permissions, com.atlassian.confluence.security.SpacePermissionGroupNamesCache, com.atlassian.confluence.user.crowd.CachedCrowdMembershipDao.STRING_PARENT_CACHE, com.atlassian.confluence.user.crowd.CachedCrowdUserDao.USER_CACHE, com.atlassian.confluence.user.crowd.DefaultApplicationCache]
   -- referer: http://quicksilver:8090/confluence/questions/1048793/tete | url: /confluence/cq/answerquestion.action | traceId: 28211703339576ca | userName: guivianabr2
  2017-01-13 12:29:45,289 ERROR [AtlassianEvent::CustomizableThreadFactory-2] [plugins.questions.notification.QuestionsNotificationsSender] sendNotification Error sending notification
   -- url: /confluence/cq/answerquestion.action | traceId: 28211703339576ca | userName: guivianabr2 | referer: http://quicksilver:8090/confluence/questions/1048793/tete | action: answerquestion
  java.lang.IllegalArgumentException: parameter value cannot be null
  	at org.springframework.util.Assert.notNull(Assert.java:115)
  	at com.atlassian.confluence.web.UrlBuilder.add(UrlBuilder.java:77)
  	at com.atlassian.confluence.plugins.questions.notification.QuestionsNotificationsSender.getWebItemLinks(QuestionsNotificationsSender.java:279)
  	at com.atlassian.confluence.plugins.questions.notification.QuestionsNotificationsSender.access$600(QuestionsNotificationsSender.java:54)
  	at com.atlassian.confluence.plugins.questions.notification.QuestionsNotificationsSender$1.call(QuestionsNotificationsSender.java:206)
  

• If the question's owner answers his question, he will receive the notification, however, from other users, he won't.

Notes

UserA receives notifications of his question posted (question created).
UserA receives notifications of comments added into an answer.

Enabling "Notify of my own changes" at users' profile level does not affect this.

Workaround

none so far