I have exactly the same problem, but it works from times to times ... (actually, it’s works longer than it bugs)
I get fed up with this pb so I unset the Captcha, because I don’t want my customers to be unable to reach me.
If I have too many spams, I will reset it again.
I hope it will be OK until I found a solution.
Does any one have an idea about what’s happening ? It seams to be linked with the server ...
note : I use MODx with infomaniak
Thank you all