Maybe you configured the spamblock to include the 'replyto' field as a required field?
If freeing this doesn't help, you might have a cache compile issue. Please empty the templates_c directory or the specific contactform file in it.

Sorted needed to remote replyto from spamblocker and then purge ...

Guys

First off thanks for great software, I have been using serendipity for quite a few years now and it has been pretty solid.

I have just moved my serendipity blog to another server and upgraded it from 1.2 to 1.6.

Everything is working great except the Contact Form.

Any attempt to use it ...