Full trust is required indeed, because our Workflow service is based on Microsoft's Workflow Foundation which requires a Full trust server environment. Unfortunately Medium trust
is not supported because of this.
Since you are still experiencing issues with your shared hosting provider I can suggest you is to navigate to our Hosting Partners
page and choose a hosting parting from the list - they are certified to run Sitefinity without having you to experience such errors.
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items