This is a very common issue with many shopify store owners, when they start getting error like this below image on browsers and it stops them from being able to login to shopify admin.
Here is a quick fix for it!
All you have to do is, clear the specific website cookies from your browser cache.
That will fix this immediately.
Since most of the users use Google Chrome, here is the detailed step by step process about how you can clear your specific website cookies inside Google Chrome browser.
Read it here