So, in hopes this helps you (and potentially the future masses), I ended up going down the rabbit hole of browser javascript errors after exhausting review of internal logs, etc., and have emerged victorious!
TLDR: the v6.6.1 feature addition of "dashboard search edit enhancements" may break dashboard search editing if any installed search commands/functions fail to validate syntax. Review any custom search commands/functions in searchbnf.conf and remove or fix.
In my case it was a single app, "Splunk Query for Graphite" aka Carbonmine where the searchbnf.conf contains a typo in the "sytanx"...rather "syntax" parameter. This was not an easy item to identify as my browser was throwing errors which indicated dozens of potential problems none of which were the issue (I spent hours troubleshooting reverse proxy caching and auth tokens in vain).
Ultimately, in my case, it was the volume of POST requests coming back as failed that upon inspection showed the browser is attempting to validate each search parameter (for each search on dashboard...which generated a ton of errors). The part that is awful is Splunk is validating against EVERY command available to the user regardless of if it is used in the search! So even though NONE of my searches were using carbonmine's search commands, the entire feature crashed editing!
The current v6.6.1 common.js and mode-spl.js apparently do not handle these failures well and simply fail to recover. This in turn causes the behavior observed where you can edit a single panel's search configuration (though syntax highlighting will likely not work) but subsequent attempts fail to even load the edit modal.
