I tested your sent sample with WebInput 4.0 build 3 and WebUI.NET Framework 3.0 build 752 on Internet Explorer 8.0.6001.18882 and Mozilla Firefox 3.6. Both of those assembly files are the nightly-build version of WebInput 4.0 and WebUI.NET Framework 3.0. After I follow the steps to reproduce the issue carefully mentioned in your first post (entering “a” in both WebInput and click Sign In and then click Submit), there is no error I found when view the sample in IE 8 or Mozilla Firefox 3.6.

I enclosed the same build number of nightly-build hotfix of WebInput 4.0 and WebUI.NET Framework 3 that I use on my test project. Please have the nightly-build hotfix tested on your end and let us know whether it helps or not; or if you have different result.

Disclaimer: the attached hotfix is a nightly-build hotfix and was not tested for regression. I hope you can collaborate and test it in your project.