I'm trying to upgrade an existing Umbraco installation from 7.1.4 to 7.4.2. The installer executed just fine, however, when starting the site or Umbraco, the following exception occurs. It appears to have something to do with indexing members (which are present in the installation). Anyone has a clue where to start?

  [InvalidOperationException: Sequence contains more than one matching element]
   System.Linq.Enumerable.SingleOrDefault(IEnumerable`1 source, Func`2 predicate) +349
   Umbraco.Core.Persistence.Repositories.VersionableRepositoryBase`2.GetPropertyCollection(Sql docSql, IEnumerable`1 documentDefs) +3345
   Umbraco.Core.Persistence.Repositories.MemberRepository.ProcessQuery(Sql sql) +682
   Umbraco.Core.Persistence.Repositories.VersionableRepositoryBase`2.GetPagedResultsByQuery(IQuery`1 query, Int64 pageIndex, Int32 pageSize, Int64& totalRecords, Tuple`2 nodeIdSelect, Func`2 processQuery, String orderBy, Direction orderDirection, Func`1 defaultFilter) +2091
   Umbraco.Core.Persistence.Repositories.MemberRepository.GetPagedResultsByQuery(IQuery`1 query, Int64 pageIndex, Int32 pageSize, Int64& totalRecords, String orderBy, Direction orderDirection, String filter) +811
   Umbraco.Core.Services.MemberService.GetAll(Int64 pageIndex, Int32 pageSize, Int64& totalRecords) +155
   Umbraco.Core.Services.MemberService.GetAll(Int32 pageIndex, Int32 pageSize, Int32& totalRecords) +29
   UmbracoExamine.UmbracoMemberIndexer.PerformIndexAll(String type) +446
   UmbracoExamine.BaseUmbracoIndexer.IndexAll(String type) +35
   UmbracoExamine.BaseUmbracoIndexer.PerformIndexRebuild() +112
   UmbracoExamine.UmbracoContentIndexer.RebuildIndex() +86
   Umbraco.Web.WebBootManager.Complete(Action`1 afterComplete) +407
   Umbraco.Core.UmbracoApplicationBase.StartApplication(Object sender, EventArgs e) +364

[HttpException (0x80004005): Sequence contains more than one matching element]
   System.Web.HttpApplicationFactory.EnsureAppStartCalledForIntegratedMode(HttpContext context, HttpApplication app) +544
   System.Web.HttpApplication.RegisterEventSubscriptionsWithIIS(IntPtr appContext, HttpContext context, MethodInfo[] handlers) +186
   System.Web.HttpApplication.InitSpecial(HttpApplicationState state, MethodInfo[] handlers, IntPtr appContext, HttpContext context) +172
   System.Web.HttpApplicationFactory.GetSpecialApplicationInstance(IntPtr appContext, HttpContext context) +402
   System.Web.Hosting.PipelineRuntime.InitializeApplication(IntPtr appContext) +343

[HttpException (0x80004005): Sequence contains more than one matching element]
   System.Web.HttpRuntime.FirstRequestInit(HttpContext context) +579
   System.Web.HttpRuntime.EnsureFirstRequestInit(HttpContext context) +112
   System.Web.HttpRuntime.ProcessRequestNotificationPrivate(IIS7WorkerRequest wr, HttpContext context) +716