As already mentioned by Cloudar, clearing cookies are one of the solutions. Once the cookies are cleared, the session is expired and it redirects successfully to the login page. In case, you don't want to clear the cookies, you can visit <custom_domain>/home/signout.do to log out the existing session and load the login page.
Another solution that can be suggested here is
If the End-Customer is already logged in, you can directly visit <custom_domain>/app/client to access the client instead of loading the Login Page.
We will try to simplify this and make the user experience for MSP End-Customers better at our end.