Facing this issue, I have both Solr 7.2.1 with 8983 and Solr 6.6.2 8984 on my machine. I am running 1 Solr Service at a time. When I start 6.6.2 it works fine for Sitecore 9.0.2, whereas my 9.1 instance is using solr 7.2.1 so I have to run solr 7.2.1 and stop the solr 6.6.2. I have copied the 6.6.2 cores to the 7.2.1 so I can use only 7.2.1 for my Sitecore 9.0.2.

Strange Behavior Once I am logged in and switch to solr 7.2.1, it works fine. I logged out and re logged in a well and works fine.

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