I have faced the same issue.
My report was working fine with the database source in which I have designed it. Then we decided to use different database name. I was providing the database name through config file, once I changed this database name I started getting the same error as you mentioned.
I resolved this by setting the database location as we required while executing it.
for that open ur report, select Database Expert, then select SetLocation option then create new connection and choose the database you are expecting while executing the report.
See if this works for you.........http://www.tek-tips.com/viewthread.cfm?qid=493919