sharepoint - Site Settings not working in sp 2013 after migration from sp 2010

Question

I migrated my sharepoint 2010 site to sharepoint 2013 server. The site contains custom web parts.

Now all pages (both custom and out-of-the-box) are working fine without any issue.

But when I go to site settings, I get this: An error occurred during the compilation of the requested file, or one of its dependencies. Cannot convert type 'string' to 'int'

Checking the logs, i found the following exception:

> Application error when access /_layouts/settings.aspx,
Error=c:\Windows\Microsoft.NET\Framework64\v4.0.30319\Temporary ASP.NET Files\root\e75b3e8d\191e7f\App_Web_settings.aspx.2a428413.q0ornjvx.0.cs(491):
error CS0030: Cannot convert type 'string' to 'int'
   at System.Web.Compilation.AssemblyBuilder.Compile()
   at System.Web.Compilation.BuildProvidersCompiler.PerformBuild() 
   at System.Web.Compilation.BuildManager.CompileWebFile(VirtualPath virtualPath)  
   at System.Web.Compilation.BuildManager.GetVPathBuildResultInternal(VirtualPath virtualPath, Boolean noBuild, Boolean allowCrossApp, Boolean allowBuildInPrecompile, Boolean throwIfNotFound, Boolean ensureIsUpToDate)  
   at System.Web.Compilation.BuildManager.GetVPathBuildResultWithNoAssert(HttpContext context, VirtualPath virtualPath, Boolean noBuild, Boolean allowCrossApp, Boolean allowBuildInPrecompile, Boolean throwIfNotFound, Boolean ensureIsUpToDate)  
   at System.Web.Compilation.BuildManager.GetVirtualPathObjectFactory(VirtualPath virtualPath, HttpContext context, Boolean allowCrossApp, Boolean throwIfNotFound)  
   at System.Web.Compilation.BuildManager.CreateInstanceFromVirtualPath(VirtualPath virtualPath, Type requiredBaseType, HttpContext context, Boolean allowCrossApp)  
   at System.Web.UI.PageHandlerFactory.GetHandlerHelper(HttpContext context, String requestType, VirtualPath virtualPath, String physicalPath)  
   at System.Web.HttpApplication.MaterializeHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()  
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)
> 
> System.Web.HttpCompileException: c:\Windows\Microsoft.NET\Framework64\v4.0.30319\Temporary ASP.NET Files\root\e75b3e8d\191e7f\App_Web_settings.aspx.2a428413.q0ornjvx.0.cs(491):
 error CS0030: Cannot convert type 'string' to 'int'    at System.Web.Compilation.AssemblyBuilder.Compile()
     at System.Web.Compilation.BuildProvidersCompiler.PerformBuild() 
     at System.Web.Compilation.BuildManager.CompileWebFile(VirtualPath virtualPath)
     at System.Web.Compilation.BuildManager.GetVPathBuildResultInternal(VirtualPath virtualPath, Boolean noBuild, Boolean allowCrossApp, Boolean allowBuildInPrecompile, Boolean throwIfNotFound, Boolean ensureIsUpToDate)
     at System.Web.Compilation.BuildManager.GetVPathBuildResultWithNoAssert(HttpContext context, VirtualPath virtualPath, Boolean noBuild, Boolean allowCrossApp, Boolean allowBuildInPrecompile, Boolean throwIfNotFound, Boolean ensureIsUpToDate)
     at System.Web.Compilation.BuildManager.GetVirtualPathObjectFactory(VirtualPath virtualPath, HttpContext context, Boolean allowCrossApp, Boolean throwIfNotFound)
     at System.Web.Compilation.BuildManager.CreateInstanceFromVirtualPath(VirtualPath virtualPath, Type requiredBaseType, HttpContext context, Boolean allowCrossApp)
     at System.Web.UI.PageHandlerFactory.GetHandlerHelper(HttpContext context, String requestType, VirtualPath virtualPath, String physicalPath)
     at System.Web.HttpApplication.MaterializeHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
     at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)
> 
> An error occurred during the compilation of the requested file, or one of its dependencies. Cannot convert type 'string' to 'int

'

Any help will be appreciated.

Thanks in advance, Ferdin