Up to you, if you don't want to use versions that work with each other then not much we can do about it. The bug is one version maybe calling an API entry point in the other version that doesn't exist.
Use all one version, if you can still replicate the issue then I'll track it, otherwise, try rebuilding you report and make sure you don't use NEW API's in SP 9 that may not be in version 14 runtime.
Uninstall CR for VS and then repair the BI 4.0 .NET SDK and see if it still works, replacing the version 13 assemblies in your app.config/web.config file. If it's still an issue then update BI 4.0 .NET installer to the latest SP and test again.
Also, if you are using BI 4 SDK you have a support ocntract, log a case and a Rep can work with you to resolve the runtime and issue.
Try a simple "one liner" app and that report, export it to RPT format so it has saved data and see if that works.
Good luck
Don