NOTE: This information is good as of 7/22/2015 and is subject to change.
*** Update ***
I had posted earlier that this was specific to Free users, but it has nothing to do with Free or Pro. Reposted to reflect the accurate behavior.
I was looking at the Power BI Community Site and someone had indicated that they cannot save a report.
Unable to save report
When looking into that Activity ID, it appears that we have a limit of 10 reports. This has nothing to do with if you are a Free or Pro user, but rather a resource limitation based on your Dataset/Model.
When you try to save the 11th report against the same Dataset, you get the error above. If you create an 11th report against a different Model it will work.
Adam W. Saxton | Microsoft Business Intelligence Support – Escalation Services