If you get error message "HTTP Status 400 – Invalid Path /Analytical Reporting/WebiView was requested. The request sent by the client was syntactically incorrect.", it is very likely that you are using IE 9.
IE 9 is not supported yet by BO. It is planned to be supported in BO BI 4.1 and BOE XI 3.1 SP5 that will be released somewhere in Q1 2012.