2sxc general exception errors in admin log - dotnetnuke

Question

We are encountering lot of "General Exception" errors related to DNN search and 2sxc, sometimes the site even stops loading.
I'm attaching one of the errors from the admin log, but there are plenty can anyone help?
We're using Evoq 8.5, 2sxc 8.5.4.
<log LogGUID="e8a0d5ca-12f6-4865-a184-f440611ad3d5" LogFileID="" LogTypeKey="GENERAL_EXCEPTION" LogUserID="-1" LogEventID="212842" LogUserName="" LogPortalID="-1" LogPortalName="" LogCreateDate="3/5/2017 2:02:57 AM" LogCreateDateNum="0" BypassBuffering="False" LogServerName="RD0003FF6E964E" LogConfigID="4">
<LogProperties>
<LogProperty>
<PropertyName>AbsoluteURL</PropertyName>
<PropertyValue />
</LogProperty>
<LogProperty>
<PropertyName>DefaultDataProvider</PropertyName>
<PropertyValue>DotNetNuke.Data.SqlDataProvider, DotNetNuke</PropertyValue>
</LogProperty>
<LogProperty>
<PropertyName>ExceptionGUID</PropertyName>
<PropertyValue>d5db1b63-65bb-424e-9284-d6bdeb597078</PropertyValue>
</LogProperty>
</LogProperties>
<Exception>
<AssemblyVersion />
<PortalId>-1</PortalId>
<UserId>-1</UserId>
<TabId>-1</TabId>
<RawUrl />
<Referrer />
<UserAgent />
<ExceptionHash>h7DfzrR5N2NZPGujVU3A9A==</ExceptionHash>
<Message>Search: Error while indexing module 3429 on tab 1311, portal 14</Message>
<StackTrace> at ToSic.SexyContent.Environment.Dnn7.DnnBusinessController.GetModifiedSearchDocuments(ModuleInfo moduleInfo, DateTime beginDate) in C:\Projects\2SexyContent\Web\DesktopModules\ToSIC_SexyContent\Environment\Dnn7\DnnBusinessController.cs:line 32
at DotNetNuke.Services.Search.ModuleIndexer.IndexSearchDocuments(Int32 portalId, ScheduleHistoryItem schedule, DateTime startDateLocal, Action`1 indexer)</StackTrace>
<InnerMessage>The underlying provider failed on Open.</InnerMessage>
<InnerStackTrace> at System.Data.EntityClient.EntityConnection.OpenStoreConnectionIf(Boolean openCondition, DbConnection storeConnectionToOpen, DbConnection originalConnection, String exceptionCode, String attemptedOperation, Boolean& closeStoreConnectionOnFailure)
at System.Data.EntityClient.EntityConnection.Open()
at System.Data.Objects.ObjectContext.EnsureConnection()
at System.Data.Objects.ObjectQuery`1.GetResults(Nullable`1 forMergeOption)
at System.Data.Objects.ObjectQuery`1.System.Collections.Generic.IEnumerable<T>.GetEnumerator()
at System.Linq.Enumerable.FirstOrDefault[TSource](IEnumerable`1 source)
at ToSic.Eav.BLL.EavDataController.InitZoneApp(Nullable`1 zoneId, Nullable`1 appId) in C:\Projects\eav-server\ToSic.Eav\BLL\EavDataController.cs:line 145
at ToSic.SexyContent.Internal.AppManagement.EnsureAppIsConfigured(Int32 zoneId, Int32 appId, String appName) in C:\Projects\2SexyContent\Web\DesktopModules\ToSIC_SexyContent\SexyContent\Internal\AppManagement.cs:line 106
at ToSic.SexyContent.App.InitializeResourcesSettingsAndMetadata(Boolean allowSideEffects) in C:\Projects\2SexyContent\Web\DesktopModules\ToSIC_SexyContent\SexyContent\App.cs:line 110
at ToSic.SexyContent.App..ctor(Int32 zoneId, Int32 appId, PortalSettings ownerPortalSettings, Boolean allowSideEffects) in C:\Projects\2SexyContent\Web\DesktopModules\ToSIC_SexyContent\SexyContent\App.cs:line 103
at ToSic.SexyContent.ContentBlock.ModuleContentBlock..ctor(ModuleInfo moduleInfo, IEnumerable`1 overrideParams) in C:\Projects\2SexyContent\Web\DesktopModules\ToSIC_SexyContent\SexyContent\ContentBlock\ModuleContentBlock.cs:line 55
at ToSic.SexyContent.Search.SearchController.GetModifiedSearchDocuments(ModuleInfo moduleInfo, DateTime beginDate) in C:\Projects\2SexyContent\Web\DesktopModules\ToSIC_SexyContent\SexyContent\Search\SearchController.cs:line 48
at ToSic.SexyContent.Environment.Dnn7.DnnBusinessController.GetModifiedSearchDocuments(ModuleInfo moduleInfo, DateTime beginDate) in C:\Projects\2SexyContent\Web\DesktopModules\ToSIC_SexyContent\Environment\Dnn7\DnnBusinessController.cs:line 28</InnerStackTrace>
<Source>ToSic.SexyContent</Source>
<FileName />
<FileLineNumber>0</FileLineNumber>
<FileColumnNumber>0</FileColumnNumber>
<Method />
</Exception>
</log>

Answer 1

This sounds like a DB error which probably has nothing to do with 2sxc. I recommend to do some DB maintenance and see if it solves everything.
So it's no known issue ATM

Answer 2

Denis updated 2sxc and I've investigated the log files and found this error that might help narrow it down:
2017-11-14 12:35:46,608 [H10][Thread:76][ERROR] DotNetNuke.Services.Exceptions.Exceptions - System.Exception: [Indexing: Behavior SiteSearch (#1)] Error reading content source DNN Modules Indexer ---> ToSic.SexyContent.Search.SearchIndexException: Search: Error while indexing module 5242 on tab 1901, portal 0 ---> System.Collections.Generic.KeyNotFoundException: The given key was not present in the dictionary.
at System.ThrowHelper.ThrowKeyNotFoundException()
at System.Collections.Generic.Dictionary2.get_Item(TKey key)
at ASP._Page_Portals_0_2sxc_Documents__Environment_Filter_App_cshtml.CustomizeData() in \Portals\0\2sxc\Documents\_Environment Filter App.cshtml:line 11
at ToSic.SexyContent.Engines.RazorEngine.CustomizeData() in C:\Projects\2SexyContent\Web\DesktopModules\ToSIC_SexyContent\2Sexy Content Razor\RazorEngine.cs:line 109
at ToSic.SexyContent.Engines.EngineBase.Init(Template template, App app, ModuleInfo hostingModule, IDataSource dataSource, InstancePurposes instancePurposes, SxcInstance sxcInstance, Log parentLog) in C:\Projects\2SexyContent\Web\DesktopModules\ToSIC_SexyContent\SexyContent\Engines\EngineBase.cs:line 64
at ToSic.SexyContent.Search.SearchController.GetModifiedSearchDocuments(ModuleInfo moduleInfo, DateTime beginDate) in C:\Projects\2SexyContent\Web\DesktopModules\ToSIC_SexyContent\SexyContent\Search\SearchController.cs:line 143
at ToSic.SexyContent.Environment.Dnn7.DnnBusinessController.GetModifiedSearchDocuments(ModuleInfo moduleInfo, DateTime beginDate) in C:\Projects\2SexyContent\Web\DesktopModules\ToSIC_SexyContent\Environment\Dnn7\DnnBusinessController.cs:line 85
--- End of inner exception stack trace ---
at ToSic.SexyContent.Environment.Dnn7.DnnBusinessController.GetModifiedSearchDocuments(ModuleInfo moduleInfo, DateTime beginDate) in C:\Projects\2SexyContent\Web\DesktopModules\ToSIC_SexyContent\Environment\Dnn7\DnnBusinessController.cs:line 89
at DnnSharp.Common2.Services.Dnn.ModuleService.<GetSearchItems>d__4.MoveNext()
at DnnSharp.SearchBoost.Core.ContentSource.DnnModules.<GetIndexingJobsForTabModules>d__26.MoveNext()
at DnnSharp.SearchBoost.Core.ContentSource.DnnModules.<GetIndexingJobs>d__25.MoveNext()
at DnnSharp.SearchBoost.Core.ContentSource.DnnModules.<Query>d__20.MoveNext()
at DnnSharp.SearchBoost.Core.Services.QueryContentSourcesService.QueryAllContentSourcesToCache(SearchBehavior behavior, List1 cache, CancellationTokenSource cancellationToken)
--- End of inner exception stack trace ---