I faced the issue above when I upgraded from Sitecore 8.1 update 3 to Sitecore 8.2 update 1.
I only got this exception where I use configuration factory for pipeline because of Dependency Injection. Like the following:
|<processor patch:before="*[@type='Sitecore.Pipelines.HttpRequest.QueryStringResolver, Sitecore.Kernel']"|
Then I got the exception “No group node for sub node: httpRequestBegin”. After I digged into the new Sitecore.Kernel I found that Sitecore wants a group ancestor for the processor. That did not really help me because if I put the group node above a processor then Sitecore does not know that it is a processor.
So I deciced to report this is a bug and they provided me an answer that they will provide a hotfix package for this issue. The reference number is 142817.
We got the hotfix from Sitecore support. They had to do changes in Sitecore.Kernel assembly. The fix is only working for Sitecore 8.2 update 1 that’s why I don’t want to share here. Good news that they are going to include this fix in Sitecore 8.2 update 3.