I cannot reproduce this problem using medium trust.
Make sure you are using the Web.config file that shipped with mojoPortal., if you upgraded don't try to keep the old one, back it up and restore customizations to the new file. Don't make any changes in the globalization element in Web.config.
Try changing the editor in site settings and see if that makes a difference.
Make sure you are using the .NET 4 version of mojoPortal and of Form Wizard.
If all else fails I would try re-deploying all the files.
That method GetDefaultCulture that is logging this error is used anywhere an editor is used so it is not specific to Form Wizard, and even if it logs that error it should not make a blank page because it still returns the current culture even if it handles that error. That is why the error says it swallowed the error, in other words it handled that error.
Hope it helps,