It seems that your web hosting security policy does not allow you to run Sitefinity in a proper way. Probably you need to run your website in Medium Trust Support
. Check out our UserManual
. You need to have read and write permissions for App_Data folder where the database resides. If your web hosting does not support standard configuration for trust level (medium) you can ask them for more information.
Let us know if there is anything else that we can do for you.
the Telerik team