EPiServer Find error on upgrade to 12.0.0.4448

Hi,

I'm trying to upgrade to the latest version of Find on an existing commerce site. It worked fine before the Find upgrade, now I get this:

[ReflectionTypeLoadException: Unable to load one or more of the requested types. Retrieve the LoaderExceptions property for more information.]
System.Reflection.RuntimeModule.GetTypes(RuntimeModule module) +0
System.Reflection.Assembly.GetTypes() +118
EPiServer.Find.ClientConventions.<>c__DisplayClass5.b__3(Assembly a) +30
EPiServer.Find.Helpers.ListExtensions.ForEach(IEnumerable`1 value, Action`1 action) +154
EPiServer.Find.ClientConventions.NestedConventions.MarkAllImplementationsAsNested(Type declaringType, String name) +209
EPiServer.Find.Commerce.CatalogContentClientConventions.ApplyNestedConventions(NestedConventions nestedConventions) +298
EPiServer.Find.Commerce.CatalogContentClientConventions.ApplyConventions(IClientConventions clientConventions) +81
EPiServer.Find.Commerce.FindCommerceInitializationModule.Initialize(InitializationEngine context) +104
EPiServer.Framework.Initialization.Internal.ModuleNode.Execute(Action a, String key) +56
EPiServer.Framework.Initialization.InitializationEngine.InitializeModules() +244

[InitializationException: Initialize action failed for Initialize on class EPiServer.Find.Commerce.FindCommerceInitializationModule, EPiServer.Find.Commerce, Version=9.6.0.138, Culture=neutral, PublicKeyToken=8fe83dea738b45b7]
EPiServer.Framework.Initialization.InitializationEngine.InitializeModules() +752
EPiServer.Framework.Initialization.InitializationEngine.ExecuteTransition(Boolean continueTransitions) +153
EPiServer.Framework.Initialization.InitializationModule.EngineExecute(HostType hostType, Action`1 engineAction) +786
EPiServer.Framework.Initialization.InitializationModule.FrameworkInitialization(HostType hostType) +230
EPiServer.Global..ctor() +104
SomeApp.Web.EPiServerApplication..ctor() +43

I'm using CMS v9.8.3

Any idea what's going wrong?

Many thanks.