Hi, I was having the same problem. Finally, what solved it was som,ething extremely simple. Hopefully, your problem is easily fixed, too.
When I wanted to go to the manager in the browser, I added the “www” in the url, something the directions says not to do.
So, I had to use
https://www.myblog.com/wp-plugin-mgr.php
and not
https://myblog.com/wp-plugin-mgr.php
(as dr. Dave outlines in the README).
Hope this helps. It’s a great plugin!
Neil