I am using custom master page and installed the nintex forms 2013 in my sharepoint 2013 environment.
When I click on any New Item which contains the Nintex form getting below error.
The control collection cannot be modified during DataBind, Init, Load, PreRender or Unload phases.
Fixed It...
Open custom master html file...
get this from header:
<!--SPM:<SharePoint:AjaxDelta id="DeltaSPWebPartManager" runat="server">-->
<!--SPM:<WebPartPages:SPWebPartManager runat="server"/>-->
<!--SPM:</SharePoint:AjaxDelta>-->
Cut it out...
go to the body part... find div with the id "s4-workspace" and paste the code before this div like this:
[...]
<!--SPM:<SharePoint:AjaxDelta id="DeltaSPWebPartManager" runat="server">-->
<!--SPM:<WebPartPages:SPWebPartManager runat="server"/>-->
<!--SPM:</SharePoint:AjaxDelta>-->
<div id="s4-workspace" class="s4-nosetwidth">
[...]
Hope this helps!