When running load tests this exceptions happens in this line:

at EPiServer.Forms.Helpers.Internal.FormsExtensions.ToJson(Object value)
   at AspNetCoreGeneratedDocument.FormsViews_Views_ElementBlocks_FormContainerInitScript.<ExecuteAsync>d__0.MoveNext()
   at Microsoft.AspNetCore.Mvc.Razor.RazorView.<RenderPageCoreAsync>d__20.MoveNext()
   at Microsoft.AspNetCore.Mvc.Razor.RazorView.<RenderPageAsync>d__19.MoveNext()
   at Microsoft.AspNetCore.Mvc.Razor.RazorView.<RenderAsync>d__18.MoveNext()
   at Microsoft.AspNetCore.Mvc.ViewFeatures.HtmlHelper.<RenderPartialCoreAsync>d__60.MoveNext()
   at Microsoft.AspNetCore.Mvc.ViewFeatures.HtmlHelper.<PartialAsync>d__57.MoveNext()
   at AspNetCoreGeneratedDocument.FormsViews_Views_ElementBlocks_Components_FormContainerBlock_FormContainerBlock.<>c__DisplayClass0_0.<<ExecuteAsync>g__RenderFormBody|0>d.MoveNext()
 

Exception:

System.InvalidOperationException: 'Collection was modified; enumeration operation may not execute.'

Code executes correctly with single vu, but scaling to 2 is enough to experience this issue.