Using KMail 5.1.3 on Kubuntu 16.04.

I've got all my emails (at least since the great KMail debacle of 2010) in subfolders of a folder called "KMail Folders". Since upgrading to Ubuntu 16.04 which upgraded KMail to version 5, I can't use the "Archive Folder" on this top-level folder anymore. I can only seem to archive the subfolders, and then only one at a time. I also can't create new folders under this top level folder now. Is this a new "feature"?

I've searched around for an answer, but haven't found the right combination of keywords to locate a solution. In previous versions of KMail, I periodically archived all my KMail to a giant compressed tar archive which itself got backed (with a bunch of other personal stuff) in my offline backup.

Have I got something misconfigured, or is this the new normal? If new normal, does anyone have a recommended strategy to simplify a total archive of all kmail in one step? Maybe I was using it "wrong" before with my folder configuration and now the loophole I was exploiting is gone, but I was happy with my organization . . .

Any thoughts? Thanks!