Gateway Manager not loading under "Administrator" login

I'm wondering if anyone is having an issue logging in as "Administrator" with the recent version of the Gateway Manager app. I have an AST server running successfully for the last six years. At some point recently, something changed and I am no longer able to get to the "Settings" section in the Gateway Manager app, which is only visible when logged in as Administrator.

I can still log in as "Technician" with the default password "apple" as stated in the configuration article on GSX (but the settings (i.e. GSX Sold To, AppleID config, etc.) are not available under this user. However, when I log in as Administrator using the SAME password I've been using for years, I am presented with a blank white window, as pictured. Nothing else ever happens. I have to quit the app and relaunch it, then the same thing happens.

I have tried removing the GM app preferences, but nothing changed. The server is running 10.12.6.

Oddly, tonight I replicated the entire AST server from scratch on another Mac mini with 10.13.3 freshly installed. Re-downloaded all AST images (36GB), setup server and NetInstall/NetBoot with no issues, launched GM and the EXACT same thing happens. I can log in as Technician and view the logs, etc., but not as Administrator. No errors, no password rejection (I am using 'apple' as the password since it's a fresh config and that's what it is for the admin account, according to GSX article TP557).

I'm very confused considering this used to work fine, even under 10.12. I feel like I'm missing something simple, but creating an entirely new AST server from the ground up, I figure I would have found whatever it was. The only thing I can think of is that the GM app is different in one of the latest AST image updates and just doesn't work right, but I have no copies of an older version to test.

Thanks for any ideas

Jeff D.