I thought I'd post this as a community service.
I had seen a few threads with this subject, but none of them seemed to have helped. Some background:
I'm using the “ldaplocal auth plugin.” I didn't like the way it said "Access Denied" before you even try to log in, so I played fast-and-loose with some Apache redirects:
RedirectMatch "^$" "https://wiki.example.com/doku.php/start?do=login"
RedirectMatch "^/doku.php$" "https://wiki.example.com/doku.php/start?do=login"
I did this a few weeks ago but never correlated it with the CKG Edit save problem, and everything else on the site, including DWEdit, worked fine. I found that upon saving with ckgedit, it sends a POST to /doku.php which was breaking, thanks to my too-clever-for-my-own-good redirects. I think I have a workaround, but just wanted to write about this experience.