When I click on new calendar button in calendar overlay page, I always get this error:

Error

An unexpected error has occurred.

Troubleshoot issues with Microsoft SharePoint Foundation.

Correlation ID: 3c0a60f1-6ceb-425c-81c1-0556c43b3070

Date and Time: 5/13/2015 10:47:58 AM

I have two different calendars in my site and they both got this same error when I attempted to add new calendar overlay.

I checked LOG files message filtered by this correlation ID, this is the result:

• System.IO.FileLoadException: Loading this assembly would produce a different grant set from other instances. (Exception from HRESULT: 0x80131401)
  at Microsoft.SharePoint.Utilities.ThmxTheme.Open(Stream stream, FileMode mode, FileAccess access)
  at Microsoft.SharePoint.Utilities.ThmxTheme.Open(SPFile themeFile, Boolean readPublishedVersion, FileMode mode, FileAccess access)
  at Microsoft.SharePoint.Utilities.ThmxTheme.Open(SPSite site, String url)
  at Microsoft.SharePoint.ApplicationPages.AggregationCustomizePage.InitializeDropDownListItems()
  at Microsoft.SharePoint.ApplicationPages.AggregationCustomizePage.CreateChildControls()
  at System.Web.UI.Control.EnsureChildControls()
  at System.Web.UI.Control.PreRenderRecursiveInternal()
  at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

I'm still unsure about what is causing the error after reading the message, can someone help me to identify what's causing the error?