I've tested this with a new project, added the model reflection and excel importer modules and deployed to the sandbox. This works as expected, i can import excel files.
So the problem is probably not caused by the excel importer module, but more likely a conflict with other modules (?) in your project. Can you provide us with a test project?
Mendix 6 has been released very recently. I think it is not yet ready for Mendix 6.x. We use the Excel importer a lot in many different projects and it does work quite well.
Edit: You can check compatible modeler versions of a module or widget in the appstore. I always do a test conversion on a branch or by exporting the project and opening the export with the new version.
Arthur I think the real issue here is not that something breaks when doing a major version upgrade (5.x to 6.x), but the fact that this breaks only when deploying to a sandbox, and not locally. It should have broken locally in exactly the same way as it does when deploying to the sandbox. The fact that it did not warrants a bug report.
I removed Mendix excel importer and now its works... So this makes very unstable software, you upgrade to new version of Mendix and your App will fail... Dont like this..