So, suddenly I cannot activate the WPMU dashboard plugin on my dev sites...

I ran this through customer support chat and was unable to find a solution so I thought I'd try here.
I have quite a few development sites to set up to work on. I've set up 3 with Desktop Server, and installed the WPMU dashboard plugin no problem, and activated it, along with consequential WPMU plugins like WP Smush Pro, etc.... However, now every time I try to log in to "validate" my membership and use/download WPMU plugins, I get a white screen that says "Sorry, You don't have access to that page." Basically, I can't log in when trying to activate the WPMU dashboard plugin using my email and password, which I know is correct.

I've disabled all other plugins, removed the plugin and tried to install it other ways, have no .htaccess issues, and have looked down a few other paths with no luck.

Can anyone think of why this suddenly is happening?