Using VS2013 .net 4.5
I had this same issue.
The "Most likely causes" section on the error message page provided the most help. For me. It said "This application defines configuration in the system.web/httpModules section." Then in the "Things you can try" section it said "Migrate the configuration to the system.webServer/modules section."
<system.web>
<httpHandlers>
<add type="DevExpress.Web.ASPxUploadProgressHttpHandler, DevExpress.Web.v15.1, Version=15.1.4.0, Culture=neutral, PublicKeyToken=b88d1754d700e49a" verb="GET,POST" path="ASPxUploadProgressHandlerPage.ashx" validate="false" />
<add type="DevExpress.Web.ASPxHttpHandlerModule, DevExpress.Web.v15.1, Version=15.1.4.0, Culture=neutral, PublicKeyToken=b88d1754d700e49a" verb="GET" path="DX.ashx" validate="false" />
</httpHandlers>
<httpModules>
<add type="DevExpress.Web.ASPxHttpHandlerModule, DevExpress.Web.v15.1, Version=15.1.4.0, Culture=neutral, PublicKeyToken=b88d1754d700e49a" name="ASPxHttpHandlerModule" />
</httpModules>
</system.web>
into the system.webServer section.
<system.webServer>
<handlers>
<add type="DevExpress.Web.ASPxUploadProgressHttpHandler, DevExpress.Web.v15.1, Version=15.1.4.0, Culture=neutral, PublicKeyToken=b88d1754d700e49a" verb="GET,POST" path="ASPxUploadProgressHandlerPage.ashx" name="ASPxUploadProgressHandler" preCondition="integratedMode" />
<add type="DevExpress.Web.ASPxHttpHandlerModule, DevExpress.Web.v15.1, Version=15.1.4.0, Culture=neutral, PublicKeyToken=b88d1754d700e49a" verb="GET" path="DX.ashx" name="ASPxHttpHandlerModule" preCondition="integratedMode" />
</handlers>
<modules>
<add type="DevExpress.Web.ASPxHttpHandlerModule, DevExpress.Web.v15.1, Version=15.1.4.0, Culture=neutral, PublicKeyToken=b88d1754d700e49a" name="ASPxHttpHandlerModule" />
</modules>
</system.webServer>