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Error executing child request for handler 'System.Web.Mvc.HttpHandlerUtil+ServerExecuteHttpHandlerAsyncWrapper'.
Thanks
                
Hi Marcin,
We didn't find a solution for RenderController. We decided not to use it in an async way and honestly said, do not use them at all.  
UmbracoApiController works fine with async.
Cheers, 
                
Thanks Alex. I think that this is related to workflows, not rendering (as described in comments) and my form is not rendered yet to even think about workflows :)
Going to test on fresh installation probably or dig more into LeBlender / Forms sourcecode.
I have the same problem, and still don't have a solution... 
Can any one help me
"Renderform" inside grid

{"Error executing child request for handler 'System.Web.Mvc.HttpHandlerUtil+ServerExecuteHttpHandlerAsyncWrapper'."}   System.Web.HttpException
Error loading Partial View script (file: ~/Views/MacroPartials/InsertUmbracoFormWithTheme.cshtml)
                
I am currently having the same issue using Umbraco 7.7.3 and Umbraco Forms 6.0.3 the past 2 days. This is the error I'm getting: Error executing child request for handler 'System.Web.Mvc.HttpHandlerUtil+ServerExecuteHttpHandlerAsyncWrapper'. 
In my Visual Studio it shows a little more detail.  When the InsertUmbracoFormWithTheme macro executes and error is in the RenderAction.  Based off of the error, it is trying to use an old connection to a previous umbraco database I had created the forms on, instead of the new umbraco database(which has a different name than the previous) the forms are on now.  Any help with this would be much appreciated as I do not want to recreate the forms on the new database.
After search I found this solution 
https://github.com/umbraco/Umbraco.Forms.Issues/issues/235
Umbraco version 8
Umbraco forms version 8.2.0

                
I experienced this problem when I made async calls in my action result in my surface controller, and updating all async calls to sync fixed my problem - not happy with this solution.
Umbraco version 7.15.4 assembly: 1.0.7381.11453
                
Just had this error on our Umbraco forms 7.5.9 on an Umbraco 7.11.1 site when a form was in the grid. When using a form in an RTE outside of the grid, I was getting errors about an ambiguous reference to FileUpload.EncryptedFilePathAndFileNameSeparator even when using the vanilla Forms partials.
Problems all disappeared when I deleted the references to Microsoft.Web.Helpers from the web application.
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