This message could indicate several different problems with the reCAPTCHA service.  The following steps should be taken in order to narrow down the source of the issue:

1. Disable reCAPTCHA on the donation form and submit a test donation.  Is the donation successful without reCAPTCHA enabled?  If so, proceed to step 2.  If not, investigate BB756048 .

2. Are the Public and Private keys entered for the reCAPTCHA service correct?  This can be found in BBIS under administration, sites and settings, sites.  Update the entered keys if these have been changed. 

3. Ensure that the IP addresses for the Google Recaptcha servers are not being blocked.  Additional information can be found here:  code.google.com/p/recaptcha/wiki/FirewallsAndRecaptcha