Hi All,
Many people are facing issues when logging into BPC 10 from the Web Client. Yes, I also had gone through a lot of such issues during the first few days. So I thought, it would be helpful if I share some of the troubleshooting I have done to get this ready.
1. First of all, always start by clearing the cookies and cache of your browser. If you are using some client browser (for eg. via Citrix), clear your local browser cookies and cache also. (It is weird, but had worked for me! 
)
2. The next important thing is that, BPC 10 needs Adobe Flash Player to display the portal. So if you are getting the login screen and after entering the credentials you just see a black screen or some browser error, it would probably be the Flash Player error. You need to have latest Adobe Flash Player ActiveX and Plugin installed in your system. Just make sure you have it. (ActiveX is for Internet Explorer and Plugin is for other browsers. I had ActiveX and was using IE, but it worked for me only after installing Plugin also. Yes, weird again. And one more thing - I was using IE from remote desktop and I did not have access to install anything there. So I just tried installing in my local and bingo, it worked! ) For users who use Citrix XenApp, there is an option in the 'desktop deck' to 'Optimize Flash Content' under 'Preferences'. Please check this also.
3. Now, the next possibility is that you do not have the sufficient access to BPC Web Login. For this you need to check with your Basis / Security team whether the following two roles are added to your profile in the backend (ECC / ABAP) system.
/POA/BUI_FLEX_CLIENT
/POA/BUI_UM_USER
These two roles are mandatory for accessing BPC via web.
4. After all these are ready, and you still can't access BPC means you are missing appropriate authorizations in the system. For example, you do not have sufficient 'Environment' access. For this, you need to check with your Basis / Security Admin to assign your access. This can be done from both the frontend (BPC) and backend (ABAP) systems.
Please go through the below document for Administration from backend:
http://scn.sap.com/docs/DOC-51460
Also, here are some links to the discussion forums which could help you more.
http://scn.sap.com/thread/3494475
http://scn.sap.com/thread/3494785
Hope this helps!
Regards,
Hari