Add entry problems for some visitors

Quite a few of my visitors are having problems adding entry's and are getting an error message, invalid input.

I've tested it myself as have a couple of other people and its fine.

This has been ever since I upgraded to 2.4.1 and got the image verification.

I don't know if some people aren't seeing the image. I turned it off and straight away got spammed, so turned it back on. I want to keep the feature as its saving me deleting 100 spam messages a day.

I really don't know what the problem is. If anyone could help I'd be grateful.

My guestbook is here

We are having exactly the same problem here....

Getting "invalid input field" error.

We have had trouble replicating the error from our machines, though managed to get the error a few times.
It is however happening to many of our guests who are moaning at us that they cannot post.....

Any ideas??