Form data error when batch editing variant group products in litium 4.8.10

I have a customer who is receiving the following error when batch editing products in a variant group:

The URL-encoded form data is not valid.

   at System.Web.HttpRequest.FillInFormCollection()
   at System.Web.HttpRequest.EnsureForm()
   at System.Web.HttpRequest.get_Form()
   at System.Web.HttpRequest.get_HasForm()
   at System.Web.UI.Page.GetCollectionBasedOnMethod(Boolean dontReturnNull)
   at System.Web.UI.Page.DeterminePostBackMode()
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

As far as I can tell, the data is correct. The group has a large number of products (36). There is nothing in the log files. Any idea where I can look into the cause of the error?

Litium version: 4.8.10

@andymcpherson did you find a solution to this issue?

@marten I did not find a solution to this issue

I did, see Can't batch update variants in variant group

The page posted too much form data on submit. I had to increase aspnet:MaxHttpCollectionKeys (from 10000 to 20000).