I have both Solr 7.2.1 with port 8983 and Solr 6.6.2 with port 8984 on my machine. My 9.1 instance is using solr 7.2.1  and working fine. But my Solr 6.6.2 for Sitecore 9.0.2, is showing this error.

[![enter image description here][1]][1]

**Server Error in '/' Application.**



    Data at the root level is invalid. Line 1, position 1.
    Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 
    
    Exception Details: System.Xml.XmlException: Data at the root level is invalid. Line 1, position 1.
    
    Source Error: 
    
    An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.
    
    Stack Trace: 
    
    
    [XmlException: Data at the root level is invalid. Line 1, position 1.]
       System.Xml.XmlTextReaderImpl.Throw(Exception e) +91
       System.Xml.XmlTextReaderImpl.ParseRootLevelWhitespace() +408
       System.Xml.XmlTextReaderImpl.ParseDocumentContent() +487
       System.Xml.Linq.XDocument.Load(XmlReader reader, LoadOptions options) +60
       System.Xml.Linq.XDocument.Parse(String text, LoadOptions options) +156
       SolrNet.Impl.SolrCoreAdmin.SendAndParseHeader(ISolrCommand cmd) +40
       Sitecore.ContentSearch.SolrProvider.SolrNetIntegration.SolrPinger.Ping() +32
       Sitecore.ContentSearch.SolrProvider.SolrConnectors.SolrConnector.TestConnection() +70
       Sitecore.ContentSearch.SolrProvider.SolrStatus.OkSolrStatus() +99
       Sitecore.ContentSearch.SolrProvider.SolrStatus..cctor() +72
    
    [TypeInitializationException: The type initializer for 'Sitecore.ContentSearch.SolrProvider.SolrStatus' threw an exception.]
       Sitecore.ContentSearch.SolrProvider.SolrStatus.SetIndexForInitialization(ISearchIndex solrIndex) +0
       Sitecore.ContentSearch.SolrProvider.SolrSearchIndex.SetForReinitialization() +202
       Sitecore.ContentSearch.SolrProvider.SolrSearchIndex.InitializeSolr() +190
       Sitecore.ContentSearch.SolrProvider.SolrSearchIndex.Initialize() +46
       Sitecore.ContentSearch.SolrProvider.SolrContentSearchManager.Initialize() +169
       (Object , Object ) +9
       Sitecore.Pipelines.CorePipeline.Run(PipelineArgs args) +484
       Sitecore.Pipelines.DefaultCorePipelineManager.Run(String pipelineName, PipelineArgs args, String pipelineDomain, Boolean failIfNotExists) +236
       Sitecore.Pipelines.DefaultCorePipelineManager.Run(String pipelineName, PipelineArgs args, String pipelineDomain) +22
       Sitecore.Nexus.Web.HttpModule.Application_Start() +161
       Sitecore.Nexus.Web.HttpModule.Init(HttpApplication app) +767
       System.Web.HttpApplication.RegisterEventSubscriptionsWithIIS(IntPtr appContext, HttpContext context, MethodInfo[] handlers) +581
       System.Web.HttpApplication.InitSpecial(HttpApplicationState state, MethodInfo[] handlers, IntPtr appContext, HttpContext context) +168
       System.Web.HttpApplicationFactory.GetSpecialApplicationInstance(IntPtr appContext, HttpContext context) +277
       System.Web.Hosting.PipelineRuntime.InitializeApplication(IntPtr appContext) +369
    
    [HttpException (0x80004005): The type initializer for 'Sitecore.ContentSearch.SolrProvider.SolrStatus' threw an exception.]
       System.Web.HttpRuntime.FirstRequestInit(HttpContext context) +532
       System.Web.HttpRuntime.EnsureFirstRequestInit(HttpContext context) +111
       System.Web.HttpRuntime.ProcessRequestNotificationPrivate(IIS7WorkerRequest wr, HttpContext context) +714


  [1]: https://i.sstatic.net/qI7BC.png