I get that error when trying a quite complex report in WebI on BO 3.1 (latest patches). I know that's a very generic error.
I tried to reduce the complexity of the report step by step and found out that a sub-query, that I use in one of the data providers causes the issue. There is nothing wrong with it, but it happens only when I use the sub-query.
Do you have any idea how I could overcome that error?