Hi David ,
The default configuration for Medium Trust level does not support uploading of Zip files. Make sure you modify the application web.config file as described below:
1. Disable page and module workflow.
2. Disable health monitoring.
Sitefinity module providers cannot get their database objects created in medium trust environment and you may get errors. To prevent this you need to create at least one content item in each module before setting the trust level to medium. As I see it you are using custom module which throws the error, so you can check whether you are not using some custom logic and methods that are not allowed by you host security policy.Below are two links that describes each setting that applies to MedoimTrust level.
Default ASP.NET Policy Permissions and Trust Levels
Medium Trust Summary
I suggest that you should build the project locally, then re upload it. If this does not help compile the module as an assembly and add it to your bin. There could be some additional restrictions on Mosso that could cause the problem.
All the best,
the Telerik team