Can you post the error you get when it's directed to the 500 error page?

I have following workaround for the issue. It will fix the action url with javascript. Just add following snippet for the page. 

<script>
    (function ($) {
        // FIX Episerver xforms handling
        $(window).ready(function () {
            var $xform = $('.form.xform');
            if ($xform.length === 0) {
                return;
            }

            var action = $xform.attr('action');
            var segments = action.split('?', 2);
            var path = segments[0]; // path is invalid
            var query = segments[1];

            var currentPath = window.location.pathname; // use real path instead
            
            path = (currentPath.indexOf("XFormPost/") !== -1) ? currentPath : currentPath + 'XFormPost/';
            action = path + '?' + query;
            $xform.attr('action', action);
        });
    })(jQuery);
</script>

Fixing the url on the server side is a lot harder due to how episerver handles xforms property rendering. Therefore javascript was a quick and easy way to workaround the issue until it gets fixed by episerver.

Btw. asp.net mvc helpers Html.BeginForm and Url.Action are also flawed the same way; they too use the master language url instead of the language branch url. But these two can be fixed by setting the language manually. e.g. using own helper methods:

    public static MvcForm EpiBeginForm(this HtmlHelper htmlHelper, string actionName, string controllerName)
    {
        var currentCulture = ContentLanguage.PreferredCulture;
        var languageBranch = currentCulture.Name;
        return htmlHelper.BeginForm(actionName, controllerName, new { Language = languageBranch }, FormMethod.Post);
    }
    
    public static string EpiAction(this UrlHelper urlHelper, string actionName, string controllerName)
    {
        var currentCulture = ContentLanguage.PreferredCulture;
        var languageBranch = currentCulture.Name;
        return urlHelper.Action(actionName, controllerName, new { Language = languageBranch });
    }