Form page redirect error after update to 1.4.5/form/login/admin

After submiting the form it gives “400 - Display page not found. Please check the page exists.”
The form page have a subpage named Thankyou with that was working before the update.

I’m working on a duplicate version of Quark and this theme did have an update along with the rest of the plugins and Grav itself. Could it be it?

Thank you in advance.

For some reason its working again…
The only thing I did was remove cache from homepage (not modular) where one of the forms was…

Is there a way, after form submission, to redirect to the same page and show the alert message in there?