We're trying to upgrade an umbraco site via a nuget upgrade. It started on v7.3.0-RC, and it was later successfully upgraded to 7.3.0. Now we are getting the following errors when trying to upgrade to 7.5.8 using an existing database. Note* if we load a new project on 7.3 and upgrade to 7.5.8 it works.

Log error: 2017-02-09 10:20:05,104 [13] ERROR Umbraco.Core.DatabaseContext - Database configuration failed System.NullReferenceException: Object reference not set to an instance of an object. at Umbraco.Core.Persistence.Migrations.Upgrades.TargetVersionSevenThreeOne.UpdateUserLanguagesToIsoCode.b0(UserDto x) at System.Linq.Enumerable.WhereListIterator1.MoveNext() at Umbraco.Core.Persistence.Migrations.Upgrades.TargetVersionSevenThreeOne.UpdateUserLanguagesToIsoCode.Up() at Umbraco.Core.Persistence.Migrations.MigrationBase.GetUpExpressions(IMigrationContext context) at Umbraco.Core.Persistence.Migrations.MigrationRunner.InitializeMigrations(List1 migrations, Database database, DatabaseProviders databaseProvider, Boolean isUpgrade) at Umbraco.Core.Persistence.Migrations.MigrationRunner.Execute(Database database, DatabaseProviders databaseProvider, Boolean isUpgrade) at Umbraco.Core.Persistence.Migrations.MigrationRunner.Execute(Database database, Boolean isUpgrade) at Umbraco.Core.DatabaseContext.UpgradeSchemaAndData(IMigrationEntryService migrationEntryService) 2017-02-09 10:20:05,211 [13] ERROR Umbraco.Web.Install.Controllers.InstallApiController - Installation step DatabaseUpgrade failed. System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> Umbraco.Web.Install.InstallException: The database failed to upgrade. ERROR: The database configuration failed with the following message: Object reference not set to an instance of an object. Please check log file for additional information (can be found in '/AppData/Logs/UmbracoTraceLog.txt') at Umbraco.Web.Install.InstallSteps.DatabaseUpgradeStep.Execute(Object model) --- End of inner exception stack trace --- at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor) at System.Reflection.RuntimeMethodInfo.UnsafeInvokeInternal(Object obj, Object[] parameters, Object[] arguments) at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture) at Umbraco.Web.Install.Controllers.InstallApiController.ExecuteStep(InstallSetupStep step, JToken instruction) 2017-02-09 10:20:05,243 [13] ERROR Umbraco.Web.Install.Controllers.InstallApiController - An error occurred during installation step DatabaseUpgrade System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> Umbraco.Web.Install.InstallException: The database failed to upgrade. ERROR: The database configuration failed with the following message: Object reference not set to an instance of an object. Please check log file for additional information (can be found in '/AppData/Logs/UmbracoTraceLog.txt') at Umbraco.Web.Install.InstallSteps.DatabaseUpgradeStep.Execute(Object model) --- End of inner exception stack trace --- at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor) at System.Reflection.RuntimeMethodInfo.UnsafeInvokeInternal(Object obj, Object[] parameters, Object[] arguments) at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture) at Umbraco.Web.Install.Controllers.InstallApiController.ExecuteStep(InstallSetupStep step, JToken instruction) at Umbraco.Web.Install.Controllers.InstallApiController.PostPerformInstall(InstallInstructions installModel)