Logging into a cPanel account via "List Accounts" in WHM can sometimes return the following error.
The login is invalid.
This error is often an issue with the browser cache. To verify this, you can try reproducing the error in another browser, or a "private/incognito" window. If the error is not present there, clearing the browser cache on the browser experiencing the issue should resolve it.
See the below link for instructions on clearing the browser cache for common browsers.