Thats a very strange error. It could happen if the Web.config got corrupted during upload and the reference in the hnalder section got duplicated. It could also happpen if you had another .NET app running at the root level and mojoportal running in a sub folder. If the parent app also specifies this hanlder it could maybe cause that error.
The thing is a Web.config error should affect all pages not just the Html module. In fact the Html module does not even use a captcha for anything so it has no relation to that error.
Hope it helps,