Dynamic web applications are becoming widespread nearly in every area. ASP.NET is one of the popular development technologies in this domain. The errors in these web applications can reduce the credibility of the site and cause possible loss of a number of clients. Therefore, testing these applications becomes significant. We present an automated tool to test ASP.NET web applications against execution errors and HTML errors that cause displaying inaccurate and incomplete information. Our tool, called Mamoste, adapts concolic testing technique which interleaves concrete and symbolic execution to generate test inputs dynamically. Mamoste also considers page events as inputs which cannot be handled with concolic testing. We have performed experiments on a subset of a heavily used ASP.NET application of a government office. We have found 366 HTML errors and a faulty component which is used almost every page in this application. In addition, Mamoste discovered that a common user control is misused in several dynamically generated pages.