I have a SharePoint 2013 site and when i try to deploy a web-part or custom aspx page with Ext.Net I received this error message "Sys.ArgumentException: The format string is invalid." This error was reported before and been solved in the ext.js check the solution here
I tried the workarounds but it didn't work. Seems the issue is still exist when Ext.NET is used with SharePoint2013.
Please advice