Sitecore Web Forms For Marketers 8.1 rev.151217 (Update-1) has recently been released, see release notes here As mentioned in the release notes this includes a new feature to prevent forging request.
For some reason I’m seeing valid request (the initial form was served by the Sitecore server and is submitted back to the same server) incorrectly being detected as forged request which causes the form the error. I’m currently working with Sitecore to try to understand why this is happening and understand what the root cause is. I will let you know if i figure out this issue.
Please let me know if anyone else is running into the same issue hopefully it can help troubleshoot this. Also Sitecore can provide a custom DLL which avoids the page crashing in this scenario.
Jeroen's Sitecore Blog 
Hi. I’m having the same problem. Did you get an answer from Sitecore?
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Hi, I’ve not had time to look into this further due to other priorities. However Sitecore has provided us a custom DLL as a workaround. The issue id for this is 456464 I’m sure they can provide you same DLL. Just curious what session state provider are you using?
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Hello Jeroen. I am facing the same issue in Update-2. Could you please provide me a dll and patch you get from support? I have asked them, but for now I don’t have response.
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Hi Radek, sorry for delay. Do you still need DLL and patch? Did you happen to ever figure out the issue? Thanks!
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