Do you have any custom code on the submit button that may interfere with the FormsMvcRouteHandler?

The code has not been change since the upgrade, so no costom code, as far as i am aware:

<button type="submit" name="submit" value="<%: SubmitButtonType.NextStep.ToString() %>" class="Form__NavigationBar__Action FormExcludeDataRebind btnNext" <%: nextButtonDisableState %>><%: Html.Translate("/episerver/forms/viewmode/stepnavigation/next")%></button>

if you browse to given link directly - what's being returned?

Please show the description of error 500 along with stack trace, so we can detech what wrong with your forms.

valdis iljuconoks if i navigate to the webResource.asxd?

i get: 

(function(n){if(typeof epi=="undefined"||typeof epi.EPiServer=="undefined"||typeof epi.EPiServer.Forms=="undefined"){console.error("Forms is not initialized correctly");return}if(typeof n=="undefined"){console.error("Forms cannot work without jQuery");return}var t=epi.EPiServer.Forms.Utils,i=epi.EPiServer.Forms.Data,r=epi.EPiServer.Forms.Extension,u=epi.EPiServer.Forms.Validation,f=epi.EPiServer.Forms.Navigation;epi.EPiServer.Forms.__DebounceTimer=null;epi.EPiServer.Forms.__Initialized=!(epi.EPiServer.Forms.__Initialized===undefined);n.extend(!0,epi.EPiServer.Forms,{Utils:{debounce:function(n,t){return function(){var i=this,r=arguments;clearTimeout(this.__DebounceTimer);this.__DebounceTimer=setTimeout(function(){n.apply(i,r)},t)}},loadExternalScriptOnDemand:function(n,t,i){for(var o=document.getElementsByTagName("head")[0],u=0,f=n.length,e=null,r;u<f;u++)e=n[u],r=document.createElement("script"),r.type="text/javascript",r.async=i||!1,r.defer=i||!1,r.src=e,o.appendChild(r),u===f-1&&this._setupCallback(r,t)},loadExternalCssOnDemand:function(n,t){for(var e=document.getElementsByTagName("head") ...

Dac Thach Nguyen 
Below is the Error log when the form is sumbited:

2018-04-24T07:33:10,Error,tsfg01ze518inte,ab2877,636601519900979825,0,9424,9,"EPiServer.Forms.Internal.Security.DefaultAntiForgeryValidator : Failed to validate the anti-forgery token

System.Web.Mvc.HttpAntiForgeryException (0x80004005): The required anti-forgery cookie "__RequestVerificationToken" is not present.

   at System.Web.Helpers.AntiXsrf.TokenValidator.ValidateTokens(HttpContextBase httpContext, IIdentity identity, AntiForgeryToken sessionToken, AntiForgeryToken fieldToken)

   at System.Web.Helpers.AntiXsrf.AntiForgeryWorker.Validate(HttpContextBase httpContext, String cookieToken, String formToken)

   at EPiServer.Forms.Internal.Security.DefaultAntiForgeryValidator.Validate(HttpContextBase httpContext)",

if you are overriding forms block template, then you might be missing anti-forgery token added to the request.

<%= Html.GenerateAntiForgeryToken(Model) %>

Thanks this worked valdis iljuconoks