Unpaid Expenses - attempting to Save or complete - Requested Operation Fails

Attempting to Save a new row for an Unpaid Invoice fails today - was working yesterday.

Elmah records -

System.Net.WebException

The remote server returned an error: (400) Bad Request.
System.Net.WebException: The remote server returned an error: (400) Bad Request.
at Mx.Financial.Services.CommandServices.EndOfMonthStatusCheckCommandService.CallCfaEomEndPointForUnpaidExpenses(EndOfMonthApiAttributesRequest apiAttributes)
at System.Web.Http.Controllers.ReflectedHttpActionDescriptor.ActionExecutor.<>c_DisplayClassc.<GetExecutor>b_6(Object instance, Object[] methodParameters)
at System.Web.Http.Controllers.ReflectedHttpActionDescriptor.ExecuteAsync(HttpControllerContext controllerContext, IDictionary`2 arguments, CancellationToken cancellationToken)
— End of stack trace from previous location where exception was thrown —
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Web.Http.Controllers.ApiControllerActionInvoker.<InvokeActionAsyncCore>d__0.MoveNext()
— End of stack trace from previous location where exception was thrown —