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Have you by any chance overriden FormContainerInitScript.cshtml file by placing it in your project ? The package adds some antiforgery stuff in the file (epi.EPiServer.SystemSettings at the bottom) you might be missing after the upgrade. Adding that section manually should fix the issue
Oooh, yeah I have and since it was a minor update I didn't think about checking for new updated views.
You might get the console warning, not error, as shown below. You will see it when the form is valid and you try to submit it. Then check you FormContainerInitScript as well.
I've just upgraded from EpiServer.Forms 5.8.0 to EpiServer.Forms 5.8.2 and now I get this error message when trying to submit a form.
These are the form packages that my project is using and I've upgraded each one by one to test which one is the culprit and after upgrading Episerver.Forms to 5.8.2 this error occurs (after upgrading both .Core and .Ui to 5.8.2). On 5.8.0 it works as expected.