I have a similar issue: I created a pdf form, tested it by filling it in for myself - all fine, but I wasn't getting any responses in. So, not knowing what else to do, I deleted the form, set it up again - now I can't even fill it in for myself anymore. I am getting the same error "form can no longer be submitted".
Also, renaming the form didn't help.
When you say that "you were not getting any responses" do you mean that people told you they were submitting but you were not getting anything in the response table?
Once you delete the form in FormsCentral you automatically break all distributed PDF forms or distributed online fillable form link. You need to redistribute your new PDF form and resend your new online form's link.