This issue only occurs on master pages, as Sitefinity has to work around the case where the entire page is wrapped in a <form> element. For pure MVC pages (ones based on MVC templates) all Sitefinity does is insert its own <form> tag, and this issue does not occur.
See where querySelectorAll is being used with regards to the document’s forms? It is depending on your master page <form> to have an ID of “aspnetForm”. When I create new master pages in Visual Studio, it tends to default the <form>’s ID to “form1”, which leads into this BeginFormSitefinity-related error a lot of the time.
The error you run into looks something like this. It occurs when the page is loaded, and trying to submit the form just causes a page refresh with no action taken:
“Uncaught TypeError: Cannot read property ‘querySelectorAll’ of undefined”
At the time of writing, while Progress has acknowledged that this is a bug, there is still no fix, completed or scheduled, for it. Until Progress fixes this issue, I would recommend always coding your master pages so that the <form>’s ID is set to “aspnetForm” out of the gate. Even if you’re not yet using Html.BeginFormSitefinity, it will be best to head off this potentially-frustrating error before it happens.