Could not load file or assembly

We are running into a "could not load file or assembly" error with our hosted application. Our developers are insistent that they have the proper dll's installed in the /BIN directory, and all required references (aspx and web.config). Our original application worked before the developers made changes. I am having the developers search their code for any changes from the prior application version that worked.

The error has changed once - before we received an error with the AjaxToolkit assembly, and now we receive an error related to a 3rd party assembly (Infragistics). I believe the developers did make a reference change or reuploaded the AjaxToolkit to the /BIN directory.

I am not sure what else to propose to the developers other than to recompile their code again after validating the references and reuploading; and, to also redownload the necessary DLLs and load to the /BIN directory again as well. The new application does work fine on localhost copy.

Any thoughts? We are stumped, and our client is losing patience with us.

Here is the error's stack trace:
[FileLoadException: Could not load file or assembly 'Infragistics35.Web.v8.3' or one of its dependencies. Strong name signature could not be verified. The assembly may have been tampered with, or it was delay signed but not fully signed with the correct private key. (Exception from HRESULT: 0x80131045)]
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 'Infragistics35.Web.v8.3' or one of its dependencies. Strong name signature could not be verified. The assembly may have been tampered with, or it was delay signed but not fully signed with the correct private key. (Exception from HRESULT: 0x80131045)]
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.WebDirectoryBatchCompiler..ctor(VirtualDirectory vdir) +163
System.Web.Compilation.BuildManager.BatchCompileWebDirectoryInternal(VirtualDirectory vdir, Boolean ignoreErrors) +53
System.Web.Compilation.BuildManager.BatchCompileWebDirectory(VirtualDirectory vdir, VirtualPath virtualDir, Boolean ignoreErrors) +175
System.Web.Compilation.BuildManager.CompileWebFile(VirtualPath virtualPath) +86
System.Web.Compilation.BuildManager.GetVPathBuildResultInternal(VirtualPath virtualPath, Boolean noBuild, Boolean allowCrossApp, Boolean allowBuildInPrecompile) +261
System.Web.Compilation.BuildManager.GetVPathBuildResultWithNoAssert(HttpContext context, VirtualPath virtualPath, Boolean noBuild, Boolean allowCrossApp, Boolean allowBuildInPrecompile) +101
System.Web.Compilation.BuildManager.GetVirtualPathObjectFactory(VirtualPath virtualPath, HttpContext context, Boolean allowCrossApp, Boolean noAssert) +126
System.Web.Compilation.BuildManager.CreateInstanceFromVirtualPath(VirtualPath virtualPath, Type requiredBaseType, HttpContext context, Boolean allowCrossApp, Boolean noAssert) +62
System.Web.UI.PageHandlerFactory.GetHandlerHelper(HttpContext context, String requestType, VirtualPath virtualPath, String physicalPath) +33
System.Web.UI.PageHandlerFactory.GetHandler(HttpContext context, String requestType, String virtualPath, String path) +37
System.Web.MaterializeHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +307
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +155

Thanks.