What does this error mean ?

What does this error mean ?

All of a sudden, its showing this error in the monitoring> Errors when I try to access application.

[1] Could not load type 'ssProject.Flows.FlowMainFlow.EntryHome'. 
   at System.Web.UI.TemplateParser.ParseString(String text, VirtualPath virtualPath, Encoding fileEncoding)
   at System.Web.UI.TemplateParser.ParseFile(String physicalPath, VirtualPath virtualPath)
   at System.Web.UI.TemplateParser.ParseInternal()
   at System.Web.UI.TemplateParser.Parse()
   at System.Web.Compilation.BaseTemplateBuildProvider.get_CodeCompilerType()
   at System.Web.Compilation.BuildProvider.GetCompilerTypeFromBuildProvider(BuildProvider buildProvider)
   at System.Web.Compilation.BuildProvidersCompiler.ProcessBuildProviders()
   at System.Web.Compilation.BuildProvidersCompiler.PerformBuild()
   at System.Web.Compilation.BuildManager.CompileWebFile(VirtualPath virtualPath)
   at System.Web.Compilation.BuildManager.GetVPathBuildResultInternal(VirtualPat...Boolean allowCrossApp, Boolean noAssert)
   at System.Web.Compilation.BuildManager.CreateInstanceFromVirtualPath(VirtualPath virtualPath, Type requiredBaseType, HttpContext context, Boolean allowCrossApp, Boolean noAssert)
   at System.Web.UI.PageHandlerFactory.GetHandlerHelper(HttpContext context, String requestType, VirtualPath virtualPath, String physicalPath)
   at System.Web.HttpApplication.MapHttpHandler(HttpContext context, String requestType, VirtualPath path, String pathTranslated, Boolean useAppConfig)
   at System.Web.HttpApplication.MapHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)
(Continues in 6d2432ba-3f35-4e6c-9582-ffeb54646087)

What could be causing this error ?
And that error is new? Check if you have any outdated reference. It's probably that.

The info the error is returning may be useful to locate it (EntryPage of MainFlow).
There is no outdated reference. I was not even working on entry page of main flow. Also when I try to publish,  It is showing following warning.

eSpace has changed
A different version of the 'XXX' eSpace is published in the server.
You may experience unexpected behavior while debugging.

How can e-space change, if I am the only one working on it ?
Your local oml is different from the one uploaded. Have you saved it from the server? The published version?
I have found the issue finally. It was totally unrelated to the error message. I was wrongly using the count output parameter of the advanced query when there was no select statement in the advacned query. Interestingly, Service studio let me publish it without showing any error message and started showing this weird issue about "Could not load type 'ssProject.Flows.FlowMainFlow.EntryHome'". When We tried to publish it from service center from a different machine, it couldn't publish and threw the error about advanced query. After fixing the issue on local service studio, it started working correctly.
Advanced queries work in misterious ways :)
It's strange that Studio hasn't warned, but at least you found the error.