I'd be happy to isolate any potential causes here. In order to do so I'll need some extra context around what your issue is.
1. Could you provide me with a screenshot with what you see on your end when attempting to login to your account from apps.shopify.com?
2. Are you receiving any specific error codes/messages?
Thanks for the update @PKEBeautySupply
I was able to confirm with our technical team that this error is commonly associated with local cookies and/or cache issues with your internet browser. I'd recommend clearing your browser's cache/cookies files. If that doesn't fix the issue, try using the incognito mode or an entirely different browser (such as Firefox or Chrome).
If you still run into the issue, please let me know. If there is anything else I can help you with, please let me know.