When the end users login, they are redirected to $sp.do instead of sp.
This behaviour is seen when $sp public page is set to inactive. All Service Portal request are routed through $sp.do. The users do not have access to when it is not active. Setting the public page to true should provide the users with the expected post-login behavior.
1)Navigate to sys_pubilc.LIST table
2)Search for $sp page.
3)Set active to True