After moving a site in 5.1 onto a server, the site loads and I'm able to login to the backoffice, but the backoffice displays a  "Server error" lightbox as soon as I login with the stack trace below.

I originally had problems with duplicate routes which was caused by dlls being in the plugin folder and the bin folder. After removing the duplicates from the plugin folder the sites loads but with the backoffice error. 

Any help would be appreciated.

[NullReferenceException: Object reference not set to an instance of an object.]
   Umbraco.Cms.Web.Configuration.UmbracoSettings.<get_Trees>b__f(ITree x) +40
   Umbraco.Framework.DeferredEqualityComparer`1.GetHashCode(T obj) +12
   System.Linq.Set`1.InternalGetHashCode(TElement value) +49
   System.Linq.Set`1.Find(TElement value, Boolean add) +23
   System.Linq.<DistinctIterator>d__81`1.MoveNext() +259
   System.Linq.WhereEnumerableIterator`1.MoveNext() +154
   System.Linq.Buffer`1..ctor(IEnumerable`1 source) +217
   System.Linq.Enumerable.ToArray(IEnumerable`1 source) +78
   Umbraco.Cms.Web.Trees.ApplicationTreeController.Index(String appAlias) +404
   lambda_method(Closure , ControllerBase , Object[] ) +110
   System.Web.Mvc.ActionMethodDispatcher.Execute(ControllerBase controller, Object[] parameters) +17
   System.Web.Mvc.ReflectedActionDescriptor.Execute(ControllerContext controllerContext, IDictionary`2 parameters) +208
   System.Web.Mvc.ControllerActionInvoker.InvokeActionMethod(ControllerContext controllerContext, ActionDescriptor actionDescriptor, IDictionary`2 parameters) +27
   System.Web.Mvc.<>c__DisplayClass15.<InvokeActionMethodWithFilters>b__12() +55
   System.Web.Mvc.ControllerActionInvoker.InvokeActionMethodFilter(IActionFilter filter, ActionExecutingContext preContext, Func`1 continuation) +263
   System.Web.Mvc.<>c__DisplayClass17.<InvokeActionMethodWithFilters>b__14() +19
   System.Web.Mvc.ControllerActionInvoker.InvokeActionMethodFilter(IActionFilter filter, ActionExecutingContext preContext, Func`1 continuation) +263
   System.Web.Mvc.<>c__DisplayClass17.<InvokeActionMethodWithFilters>b__14() +19
   System.Web.Mvc.ControllerActionInvoker.InvokeActionMethodFilter(IActionFilter filter, ActionExecutingContext preContext, Func`1 continuation) +263
   System.Web.Mvc.<>c__DisplayClass17.<InvokeActionMethodWithFilters>b__14() +19
   System.Web.Mvc.ControllerActionInvoker.InvokeActionMethodWithFilters(ControllerContext controllerContext, IList`1 filters, ActionDescriptor actionDescriptor, IDictionary`2 parameters) +191
   System.Web.Mvc.ControllerActionInvoker.InvokeAction(ControllerContext controllerContext, String actionName) +343
   Umbraco.Cms.Web.Mvc.ActionInvokers.ControllerExtenderActionInvoker.InvokeAction(ControllerContext controllerContext, String actionName) +341
   System.Web.Mvc.Controller.ExecuteCore() +116
   System.Web.Mvc.ControllerBase.Execute(RequestContext requestContext) +97
   System.Web.Mvc.ControllerBase.System.Web.Mvc.IController.Execute(RequestContext requestContext) +10
   System.Web.Mvc.<>c__DisplayClassb.<BeginProcessRequest>b__5() +37
   System.Web.Mvc.Async.<>c__DisplayClass1.<MakeVoidDelegate>b__0() +21
   System.Web.Mvc.Async.<>c__DisplayClass8`1.<BeginSynchronous>b__7(IAsyncResult _) +12
   System.Web.Mvc.Async.WrappedAsyncResult`1.End() +62
   System.Web.Mvc.<>c__DisplayClasse.<EndProcessRequest>b__d() +50
   System.Web.Mvc.SecurityUtil.<GetCallInAppTrustThunk>b__0(Action f) +7
   System.Web.Mvc.SecurityUtil.ProcessInApplicationTrust(Action action) +22
   System.Web.Mvc.MvcHandler.EndProcessRequest(IAsyncResult asyncResult) +60
   System.Web.Mvc.MvcHandler.System.Web.IHttpAsyncHandler.EndProcessRequest(IAsyncResult result) +9
   System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +8970061
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +184