Could not load file or assembly 'System.Core , Version=2.0.5.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e, Retargetable=Yes'

I've a vimeo API dll's for uplaoding video's to Vimeo server, it works good in dev server but after moving to Live servers it doesn't work.it throws this error

{"Message":"An error has occurred.","ExceptionMessage":"Could not load file or assembly 'System.Core
, Version=2.0.5.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e, Retargetable=Yes' or one of its
 dependencies. The given assembly name or codebase was invalid. (Exception from HRESULT: 0x80131047)"
,"ExceptionType":"System.IO.FileLoadException","StackTrace":"   at System.Reflection.CustomAttribute
._CreateCaObject(RuntimeModule pModule, IRuntimeMethodInfo pCtor, Byte** ppBlob, Byte* pEndBlob, Int32
* pcNamedArgs)\r\n   at System.Reflection.CustomAttribute.CreateCaObject(RuntimeModule module, IRuntimeMethodInfo
 ctor, IntPtr& blob, IntPtr blobEnd, Int32& namedArgs)\r\n   at System.Reflection.CustomAttribute.GetCustomAttributes
(RuntimeModule decoratedModule, Int32 decoratedMetadataToken, Int32 pcaCount, RuntimeType attributeFilterType
, Boolean mustBeInheritable, IList derivedAttributes, Boolean isDecoratedTargetSecurityTransparent)\r
\n   at System.Reflection.CustomAttribute.GetCustomAttributes(RuntimeMethodInfo method, RuntimeType caType
, Boolean inherit)\r\n   at System.Reflection.RuntimeMethodInfo.GetCustomAttributes(Boolean inherit)
\r\n   at System.Web.Http.Controllers.ReflectedHttpActionDescriptor.InitializeProperties(MethodInfo methodInfo
)\r\n   at System.Web.Http.Controllers.ReflectedHttpActionDescriptor..ctor(HttpControllerDescriptor controllerDescriptor
, MethodInfo methodInfo)\r\n   at System.Web.Http.Controllers.ApiControllerActionSelector.ActionSelectorCacheItem
..ctor(HttpControllerDescriptor controllerDescriptor)\r\n   at System.Web.Http.Controllers.ApiControllerActionSelector
.<>c__DisplayClass2.<GetInternalSelector>b__0(Object _)\r\n   at System.Collections.Concurrent.ConcurrentDictionary
`2.GetOrAdd(TKey key, Func`2 valueFactory)\r\n   at System.Web.Http.Controllers.ApiControllerActionSelector
.GetInternalSelector(HttpControllerDescriptor controllerDescriptor)\r\n   at System.Web.Http.Controllers
.ApiControllerActionSelector.SelectAction(HttpControllerContext controllerContext)\r\n   at System.Web
.Http.ApiController.ExecuteAsync(HttpControllerContext controllerContext, CancellationToken cancellationToken
)\r\n   at System.Web.Http.Dispatcher.HttpControllerDispatcher.SendAsyncInternal(HttpRequestMessage request
, CancellationToken cancellationToken)\r\n   at System.Web.Http.Dispatcher.HttpControllerDispatcher.SendAsync
(HttpRequestMessage request, CancellationToken cancellationToken)"}

Is there any reason for this ? Guide me to resolve this.