DataImporter throws DatabaseHandler exception

Description

In some cases, the Import throws an exception when importing pages with a simple address (ExternalUrl).

Imports that worked without issues in a previous release (9.8.x) now throw an exception. An error occurs when importing a large export package of 240MB.

Stack trace of error:

2016-05-24 00:20:34.0782|ERROR|EPiServer.Core.Transfer.TransferLogger|10.5.3 Export/import error: [Importing content 685_690] Exception: Call from wrong thread - the DatabaseHandler classes are not thread safe!
System.InvalidOperationException: Call from wrong thread - the DatabaseHandler classes are not thread safe!
   at EPiServer.Data.Providers.SqlDatabaseHandler.AssertThreadSafe()
   at EPiServer.Data.Providers.SqlDatabaseHandler.Execute(Action action)
   at EPiServer.DataAccess.PageQuickSearchDB.FindByExternalUrl(String path)
   at EPiServer.Core.Transfer.ContentTransfer.HandleExternalUrlProperty(IContent content)
   at EPiServer.Core.Transfer.ContentTransfer.PostModifyProperties(IContent content, Boolean newContent)
   at EPiServer.Core.Transfer.ContentTransfer.Import(RawContent rawContent, AccessLevel requiredDestinationAccess, Guid& importedPageGuid)
   at EPiServer.Core.Transfer.ContentTransfer.Import(ITransferContentData content, AccessLevel requiredDestinationAccess)
   at EPiServer.Enterprise.DataImporter.ImportContents[T](XmlTextReader reader, ZipPackage package)
   at EPiServer.Enterprise.DataImporter.ImportStream(ZipPackage package, XmlTextReader reader, String partName)
   at EPiServer.Enterprise.DataImporter.ImportPartOfPackage(ZipPackage package, String partName)
   at EPiServer.Enterprise.DataImporter.ImportRaw(ZipPackage package)
   at EPiServer.Enterprise.DataImporter.Import()