I came across this issue a number of times where customers report that when they try to login to WebUI they are prompted to enter their credentials multiple times. Not only that but they also encounter warnings regarding protocol switch (HTTP to HTTPS). The resolution is quite simple. The following needs to be in place :
1. login/accept_sso2_ticket must be set to 1 for Dflt/Profile/Current
2. login/create_sso2_ticket must be set to 2 for Dflt/Profile/Current
3. Check user type in transaction SU01 -> Logon data tab, check User Type. Usually, it should be "DIALOG". If it is other types, like "SERVICE", it also will cause multiple logon requirement.
4. For warnings during login, check service crm_ui_start for CRM 7.0 and crm_ui_frame for CRM 6.0 (SICF). Open this node and click on tab Error Pages -> scroll down and click on the 'Configuration' button -> under Actions During Logon section, check Do Not Display Warnings.
This should resolve the issue.
