We still see the issue with every new instance of AEM 6.1 we start. I have a package that i took from another instance that contains the tree activation page. If you shoot me a private message, i can forward the package over to you.
Just to confirm the solution explicitly to those who are seeing this (as I did) - the root cause is that /etc/replication is missing the 'treeactivation" folder. If you export/import from a working 6.1 instance (or simply recreate the folder in CRX with correct props), the problem goes away. See attached screenshot for desired state in JCR.