Could not load file or assembly... Array subscript out of range

My asp.net project, which has previously been fine, suddenly starts doing the squiggly line under the @PAGE directive moaning about ASP.NET runtime error: could not load file or assembly... etc. I get the following error when I try and debug. I've tried all the really obvious stuff... cleaned, rebuilt, deleted temp asp.net files, deleted and readded the reference, restarted visual studio, restarted my computer etc. I'm completely stumped, and I can't find anything of any use on google. Does anyone have any idea what's going on?

Index was outside the bounds of the array. 
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.IndexOutOfRangeException: Index was outside the bounds of the array.

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: 


[IndexOutOfRangeException: Index was outside the bounds of the array.]
   System.Security.SecurityDocument.InternalGetElement(Int32& position, Boolean bCreate) +7559296
   System.Security.SecurityDocument.GetChildrenPositionForElement(Int32 position) +256
   System.Security.Policy.PolicyStatement.FromXml(SecurityDocument doc, Int32 position, PolicyLevel level, Boolean allowInternalOnly) +166
   System.Security.Policy.PolicyLevel.CheckCache(Int32 count, Char[] serializedEvidence) +94
   System.Security.Policy.PolicyLevel.Resolve(Evidence evidence, Int32 count, Char[] serializedEvidence) +70
   System.Security.PolicyManager.CodeGroupResolve(Evidence evidence, Boolean systemPolicy) +195
   System.Security.PolicyManager.ResolveHelper(Evidence evidence) +67
   System.Security.HostSecurityManager.ResolvePolicy(Evidence evidence) +32
   System.Security.PolicyManager.Resolve(Evidence evidence) +75
   System.Security.SecurityManager.ResolvePolicy(Evidence evidence, PermissionSet reqdPset, PermissionSet optPset, PermissionSet denyPset, PermissionSet& denied, Boolean checkExecutionPermission) +124
   System.Security.SecurityManager.ResolvePolicy(Evidence evidence, PermissionSet reqdPset, PermissionSet optPset, PermissionSet denyPset, PermissionSet& denied, Int32& securitySpecialFlags, Boolean checkExecutionPermission) +57

[FileLoadException: Could not load file or assembly 'MyProject.DataStore, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' or one of its dependencies. Array subscript out of range. (Exception from HRESULT: 0x80131508)]
   System.Reflection.Assembly._nLoad(AssemblyName fileName, String codeBase, Evidence assemblySecurity, Assembly locationHint, StackCrawlMark& stackMark, Boolean throwOnFileNotFound, Boolean forIntrospection) +0
   System.Reflection.Assembly.nLoad(AssemblyName fileName, String codeBase, Evidence assemblySecurity, Assembly locationHint, StackCrawlMark& stackMark, Boolean throwOnFileNotFound, Boolean forIntrospection) +43
   System.Reflection.Assembly.InternalLoad(AssemblyName assemblyRef, Evidence assemblySecurity, StackCrawlMark& stackMark, Boolean forIntrospection) +127
   System.Reflection.Assembly.InternalLoad(String assemblyString, Evidence assemblySecurity, StackCrawlMark& stackMark, Boolean forIntrospection) +142
   System.Reflection.Assembly.Load(String assemblyString) +28
   System.Web.Configuration.CompilationSection.LoadAssemblyHelper(String assemblyName, Boolean starDirective) +46

[ConfigurationErrorsException: Could not load file or assembly 'MyProject.DataStore, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' or one of its dependencies. Array subscript out of range. (Exception from HRESULT: 0x80131508)]
   System.Web.Configuration.CompilationSection.LoadAssemblyHelper(String assemblyName, Boolean starDirective) +613
   System.Web.Configuration.CompilationSection.LoadAllAssembliesFromAppDomainBinDirectory() +203
   System.Web.Configuration.CompilationSection.LoadAssembly(AssemblyInfo ai) +105
   System.Web.Compilation.BuildManager.GetReferencedAssemblies(CompilationSection compConfig) +178
   System.Web.Compilation.BuildProvidersCompiler..ctor(VirtualPath configPath, Boolean supportLocalization, String outputAssemblyName) +54
   System.Web.Compilation.CodeDirectoryCompiler.GetCodeDirectoryAssembly(VirtualPath virtualDir, CodeDirectoryType dirType, String assemblyName, StringSet excludedSubdirectories, Boolean isDirectoryAllowed) +8809426
   System.Web.Compilation.BuildManager.CompileCodeDirectory(VirtualPath virtualDir, CodeDirectoryType dirType, String assemblyName, StringSet excludedSubdirectories) +128
   System.Web.Compilation.BuildManager.CompileCodeDirectories() +265
   System.Web.Compilation.BuildManager.EnsureTopLevelFilesCompiled() +320

[HttpException (0x80004005): Could not load file or assembly 'MyProject.DataStore, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' or one of its dependencies. Array subscript out of range. (Exception from HRESULT: 0x80131508)]
   System.Web.Compilation.BuildManager.ReportTopLevelCompilationException() +58
   System.Web.Compilation.BuildManager.EnsureTopLevelFilesCompiled() +512
   System.Web.Hosting.HostingEnvironment.Initialize(ApplicationManager appManager, IApplicationHost appHost, IConfigMapPathFactory configMapPathFactory, HostingEnvironmentParameters hostingParameters) +729

[HttpException (0x80004005): Could not load file or assembly 'MyProject.DataStore, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' or one of its dependencies. Array subscript out of range. (Exception from HRESULT: 0x80131508)]
   System.Web.HttpRuntime.FirstRequestInit(HttpContext context) +8890735
   System.Web.HttpRuntime.EnsureFirstRequestInit(HttpContext context) +85
   System.Web.HttpRuntime.ProcessRequestInternal(HttpWorkerRequest wr) +259

You might check your database connections that may stopped working, index out of array may cause from it.