Getting server error In Employee workflow for enroll now screen

This is scenario specific issue and not come all time below customization:
1]QE/OE/NH has required verification both with pending.
2]Employee workflow
2]Medical Plan click on Enroll now screen approval pending pop will come
3] First click on a Close button and same time click on benadmin employee home page.
4]landing page ->next enroll now screen(dental) throwing server error.
5]after refreshing it will start workflow from Staring page not last visited page.

Refer attached screen shots and below error Log:

Exception : Invalid length for a Base-64 char array or string.
App Error Log : -
Employee ID : 2165
Action Name : EnrollNowPage
Model : System.Web.Mvc.HandleErrorInfo
Controller : UserDetails
Web Error Log : at System.Convert.FromBase64_Decode(Char* startInputPtr, Int32 inputLength, Byte* startDestPtr, Int32 destLength)
at System.Convert.FromBase64CharPtr(Char* inputPtr, Int32 inputLength)
at System.Convert.FromBase64String(String s)
at EBS.AESCryptography.DecryptString(String base64StringToDecrypt, String passphrase)
at WORKTERRA.SharedFunctionWebTier.DecryptString(String Input, Boolean EncodeURL)
at WORKTERRA.BenAdmin.Areas.UserDetails.Models.EnrollNowModel.SetEnrollNowObject(String BenefitTypeID, String BenefitTypeName)
at WORKTERRA.BenAdmin.Areas.UserDetails.Controllers.UserDetailsController.EnrollNowPage(String Parameter1, String Parameter2)
at lambda_method(Closure , ControllerBase , Object[] )
at System.Web.Mvc.ReflectedActionDescriptor.Execute(ControllerContext controllerContext, IDictionary`2 parameters)
at System.Web.Mvc.ControllerActionInvoker.InvokeActionMethod(ControllerContext controllerContext, ActionDescriptor actionDescriptor, IDictionary`2 parameters)
at System.Web.Mvc.Async.AsyncControllerActionInvoker.b__39(IAsyncResult asyncResult, ActionInvocation innerInvokeState)
at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncResult`2.CallEndDelegate(IAsyncResult asyncResult)
at System.Web.Mvc.Async.AsyncControllerActionInvoker.AsyncInvocationWithFilters.b__3d()
at System.Web.Mvc.Async.AsyncControllerActionInvoker.AsyncInvocationWithFilters.<>c_DisplayClass46.b_3f()
at System.Web.Mvc.Async.AsyncControllerActionInvoker.AsyncInvocationWithFilters.<>c_DisplayClass46.b_3f()
at System.Web.Mvc.Async.AsyncControllerActionInvoker.<>c_DisplayClass33.b_32(IAsyncResult asyncResult)
at System.Web.Mvc.Async.AsyncControllerActionInvoker.<>c_DisplayClass21.<>cDisplayClass2b.b_1c()
at System.Web.Mvc.Async.AsyncControllerActionInvoker.<>c_DisplayClass21.b_1e(IAsyncResult asyncResult)