Could not load file or assembly 'System.Reflection.TypeExtensions error in nopcommerce 3.70

I have upgraded my nopcommerce site to 3.70 and i have two websites one is static asp site and other one is nopcommerce site both is on same domain.

I have linked both websites like when customer opens my website on www.mydomain.com then displayed my static asp site and when clicks purchase button from static site then it redirects to nopcommerce site and my nopcommerce code is inside nc name folder so url for nopcommerce site is www.mydomain.com/nc

I have setup both website on single domain and its working but after some time or after some hours my nopcommerce site is shows error of Could not load file or assembly 'System.Reflection.TypeExtensions.

error: Could not load file or assembly 'System.Reflection.TypeExtensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies. The system cannot find the file specified.

I have solved this issue via replacing nop.admin dll then its working but again after some time its gives same error of Could not load file or assembly 'System.Reflection.TypeExtensions.

Please give solution if any one know about that error.