I was collaborating with my client and she was not able to access the dashboard by any means. Tried it on her Mac desktop running FF/Chrome, Mac laptop, and her business partner's Mac laptop. Could only get a blank screen. Couldn't even get to a login page to enter username/password. Just a blank screen. I was able to remote to my client's desktop and inspected the page source. Nothing but html, head, and body tags, and no other source code.
From my local side I experienced similar issues. If I changed themes, or deactivated a plugin, or updated a post/page, the page would refresh to a blank screen. Fortunately, I was able to use my back button to get to a "normal" page, and if I refreshed that page, then I could see my changes.
After two hours of trial and error, I read a few forum posts about other people having similar problems (just as recently as a week ago and going back to over 3 years ago) and the troubleshooting steps they went through. I took a cue from amongst the tweak ideas and renamed the plugin folder (to plugin.debug). Eureka! My client was able to access the dashboard! Obviously there was a plugin problem. Even though, I had deactivated some of the plugins (not all, just the most likely culprits), the issue persisted, but blasting all the plugins at once seemed to juke the dashboard. Then, I renamed the plugin folder back to the original name and refreshed the dashboard. Everything was working perfectly again. Login, logout, page/post updates, plugin setting updates, maintenance redirector... all was working perfectly.
My issue is resolved, but I'm just throwing this out there in case other's might be able to benefit from the same cue I took and save themselves some time and headache.