I am experiencing the same issue. I disabled all plugins, changed to one of the built in themes and then re-enabled the plugin. As soon as I did that (cleared my application cache from dev-tools) and refreshed the admin-page, it gave me a 404.
The only way to recover is to again disable all plugins.
This is awful, was there a recent update to WPS Hide Login? I’ve had to take down my entire site for this…
Ended up being a separate issue with an other plugin that was adding a rewrite rule to a non-existent page. Put the whole permalink structure into a weird state (best I can figure).
