It's trying to load a non-existent control because removed the old Silverlight features that didn't work anyway. When I did that, I removed the settings for these from the FeatureDefinition file for the Image Gallery but mojo doesn't remove settings when it doesn't find those settings in the FeatureDefinition file (kind of silly but can be useful).

You can run the following SQL to update your sites so they work properly:

delete from mp_ModuleDefinitionSettings
where FeatureGuid ='d572f6b4-d0ed-465d-ad60-60433893b401'
and GroupName = 'SilverlightSettings';

This will be in the next release, due soon.