Yes, I get the same error on DNN 6.1.5
'Cache Viewer' did not load correctly... DotNetNuke.Services.Exceptions.ModuleLoadException: Exception has been thrown by the target of an invocation. ---> System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> DotNetNuke.Collections.Internal.ReadLockRequiredException: Exception of type 'DotNetNuke.Collections.Internal.ReadLockRequiredException' was thrown. at DotNetNuke.Collections.Internal.SharedDictionary`2.EnsureReadAccess() at DotNetNuke.Collections.Internal.SharedDictionary`2.get_Count() --- End of inner exception stack trace --- at System.RuntimeMethodHandle._InvokeMethodFast(IRuntimeMethodInfo method, Object target, Object[] arguments, SignatureStruct& sig, MethodAttributes methodAttributes, RuntimeType typeOwner) at System.RuntimeMethodHandle.InvokeMethodFast(IRuntimeMethodInfo method, Object target, Object[] arguments, Signature sig, MethodAttributes methodAttributes, RuntimeType typeOwner) at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture, Boolean skipVisibilityChecks) at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture) at System.Reflection.RuntimePropertyInfo.GetValue(Object obj, BindingFlags invokeAttr, Binder binder, Object[] index, CultureInfo culture) at System.Reflection.RuntimePropertyInfo.GetValue(Object obj, Object[] index) at iFinity.DNN.Modules.CacheMaster.CacheController.GetCacheCollectionValues(CacheItemInfo& item, Object cacheItemValue) at iFinity.DNN.Modules.CacheMaster.CacheController.GetCacheItems(Int32 portalId) at iFinity.DNN.Modules.CacheMaster.CacheView.Page_Load(Object sender, EventArgs e) --- End of inner exception stack trace ---